A friend ask me if I can add a MP3 Player jack to his existing 1940 Ford Model 6MF490 radio. Is this possible?

Yes.
You can insert a jack on the high side of the vol control between C3 and the vol control.
The vol will still operate.

Or insert the jack on the wiper of the vol pot where it feeds the grid of the 1st audio.
Then the Mp3 player will have to be the vol control.

Will need a switched jack to cut off the radio audio when the MP3 player plug is inserted. Use shielded cable to the jack, regardless of where it is to be located. Ground the shield to the radio chassis.

Also, since MP3 output is stereo, you will need a simple resistor network to mix the two channels into one. You may also need a resistor divider to match the output level of the player to the radio's amp input. This will allow the MP3 player to be set at normal volume for earbud listening and it need not be adjusted when it is plugged into the car radio. Recommend that you use a fixed output level from the MP3 player feeding the top of the radio's volume pot- this will allow the car radio volume control to be used to set the sound level.

:Will need a switched jack to cut off the radio audio when the MP3 player plug is inserted. Use shielded cable to the jack, regardless of where it is to be located. Ground the shield to the radio chassis.
:
:Also, since MP3 output is stereo, you will need a simple resistor network to mix the two channels into one. You may also need a resistor divider to match the output level of the player to the radio's amp input. This will allow the MP3 player to be set at normal volume for earbud listening and it need not be adjusted when it is plugged into the car radio. Recommend that you use a fixed output level from the MP3 player feeding the top of the radio's volume pot- this will allow the car radio volume control to be used to set the sound level.
:
If I were doing it, I would put a SPDT switch in the high side of the volume control to completely isolate the radio from the player. A couple or resistors to tie the two channels together, and it should work very well.
Lewis

:If I were doing it, I would put a SPDT switch in the high side of the volume control to completely isolate the radio from the player. A couple or resistors to tie the two channels together, and it should work very well.
:Lewis
:

The switched jack can accomplish the radio audio cutoff. The built-in jack switch can route the demodulated radio audio to the volume control high side when no plug is inserted so that the radio can play normally; when the plug is inserted, the jack switch automatically disconnects the radio audio signal and connects the external source to the volume control. That way it's not necessary to fiddle with an extra switch- just plug in the MP3 adapter cable and go. Of course, a separate switch would be useful if it's desired to repetitively flip between the two modes, such as might be done during a storm to monitor weather warnings (although I can think of much safer places to be during a storm than inside a 74-year-old Ford).

::If I were doing it, I would put a SPDT switch in the high side of the volume control to completely isolate the radio from the player. A couple or resistors to tie the two channels together, and it should work very well.
::Lewis
::
:
:The switched jack can accomplish the radio audio cutoff. The built-in jack switch can route the demodulated radio audio to the volume control high side when no plug is inserted so that the radio can play normally; when the plug is inserted, the jack switch automatically disconnects the radio audio signal and connects the external source to the volume control. That way it's not necessary to fiddle with an extra switch- just plug in the MP3 adapter cable and go. Of course, a separate switch would be useful if it's desired to repetitively flip between the two modes, such as might be done during a storm to monitor weather warnings (although I can think of much safer places to be during a storm than inside a 74-year-old Ford).

CV:
Two good points, there.
Lewis
:

::If I were doing it, I would put a SPDT switch in the high side of the volume control to completely isolate the radio from the player. A couple or resistors to tie the two channels together, and it should work very well.
::Lewis
::
:
:The switched jack can accomplish the radio audio cutoff. The built-in jack switch can route the demodulated radio audio to the volume control high side when no plug is inserted so that the radio can play normally; when the plug is inserted, the jack switch automatically disconnects the radio audio signal and connects the external source to the volume control. That way it's not necessary to fiddle with an extra switch- just plug in the MP3 adapter cable and go. Of course, a separate switch would be useful if it's desired to repetitively flip between the two modes, such as might be done during a storm to monitor weather warnings (although I can think of much safer places to be during a storm than inside a 74-year-old Ford).

CV:
Two good points, there.
Lewis
:

:::If I were doing it, I would put a SPDT switch in the high side of the volume control to completely isolate the radio from the player. A couple or resistors to tie the two channels together, and it should work very well.
:::Lewis
:::
::
::The switched jack can accomplish the radio audio cutoff. The built-in jack switch can route the demodulated radio audio to the volume control high side when no plug is inserted so that the radio can play normally; when the plug is inserted, the jack switch automatically disconnects the radio audio signal and connects the external source to the volume control. That way it's not necessary to fiddle with an extra switch- just plug in the MP3 adapter cable and go. Of course, a separate switch would be useful if it's desired to repetitively flip between the two modes, such as might be done during a storm to monitor weather warnings (although I can think of much safer places to be during a storm than inside a 74-year-old Ford).
:
:CV:
:Two good points, there.
:Lewis
::
:
:
What ohm resistors should I use? Thanks for all the great suggestions, cant wait to try it.

For the mixer resistors, the value isn't critical, since the MP3 player output is low impedance (several ohms) feeding a high impedance volume pot (probably 500K ohms or so) so you want to pick a value that doesn't load down the output drivers, while avoiding excessive loss. 1.2 Kohms for each resistor should be fine. If you get distortion (signifying excessive player output loading) you can increase the value to 4.7K or even higher.

The resistor divider value to knock down the signal level to a suitable value requires a little experimentation to determine- it's probably easiest to use a 10K pot (linear taper) as a temp installation, then set the pot for best volume "continuity" between the MP3 player in earbud mode and in radio-source mode. Note that the two series resistors used as mixers serve as part of the voltage divider, so only one resistor from the common "mixed" node to ground is needed to complete the level attenuation.

:For the mixer resistors, the value isn't critical, since the MP3 player output is low impedance (several ohms) feeding a high impedance volume pot (probably 500K ohms or so) so you want to pick a value that doesn't load down the output drivers, while avoiding excessive loss. 1.2 Kohms for each resistor should be fine. If you get distortion (signifying excessive player output loading) you can increase the value to 4.7K or even higher.
:
:The resistor divider value to knock down the signal level to a suitable value requires a little experimentation to determine- it's probably easiest to use a 10K pot (linear taper) as a temp installation, then set the pot for best volume "continuity" between the MP3 player in earbud mode and in radio-source mode. Note that the two series resistors used as mixers serve as part of the voltage divider, so only one resistor from the common "mixed" node to ground is needed to complete the level attenuation.
:
I have a 5 pin stereo jack, you wrote one resistor from the common "mixed" node to ground is needed to complete the level attenuation" If I'm thinking correctly the jack cant be directly drilled and mounted on the chassis. The resistor completes the path to ground, right? Also a little confused on the resistor divider. Is it possible, to draw me a schematic of the circuit, If you can't post it here, my email is rc70@comcast.net
Thanks for all the help.

Sketch emailed.

:For the mixer resistors, the value isn't critical, since the MP3 player output is low impedance (several ohms) feeding a high impedance volume pot (probably 500K ohms or so) so you want to pick a value that doesn't load down the output drivers, while avoiding excessive loss. 1.2 Kohms for each resistor should be fine. If you get distortion (signifying excessive player output loading) you can increase the value to 4.7K or even higher.
:
:The resistor divider value to knock down the signal level to a suitable value requires a little experimentation to determine- it's probably easiest to use a 10K pot (linear taper) as a temp installation, then set the pot for best volume "continuity" between the MP3 player in earbud mode and in radio-source mode. Note that the two series resistors used as mixers serve as part of the voltage divider, so only one resistor from the common "mixed" node to ground is needed to complete the level attenuation.
:
MP3 Player adapter working, volume controlled by radio.
The only problem I have now is when I remove the MP3 player plug from the jack, radio plays at about half volume turned all the way down. When I turn it to increase volume, it take about half a turn then it gets louder as I turn it. I used shielded wire as instructed.
Thanks for all the help.

::For the mixer resistors, the value isn't critical, since the MP3 player output is low impedance (several ohms) feeding a high impedance volume pot (probably 500K ohms or so) so you want to pick a value that doesn't load down the output drivers, while avoiding excessive loss. 1.2 Kohms for each resistor should be fine. If you get distortion (signifying excessive player output loading) you can increase the value to 4.7K or even higher.
::
::The resistor divider value to knock down the signal level to a suitable value requires a little experimentation to determine- it's probably easiest to use a 10K pot (linear taper) as a temp installation, then set the pot for best volume "continuity" between the MP3 player in earbud mode and in radio-source mode. Note that the two series resistors used as mixers serve as part of the voltage divider, so only one resistor from the common "mixed" node to ground is needed to complete the level attenuation.
::
:MP3 Player adapter working, volume controlled by radio.
:The only problem I have now is when I remove the MP3 player plug from the jack, radio plays at about half volume turned all the way down. When I turn it to increase volume, it take about half a turn then it gets louder as I turn it. I used shielded wire as instructed.
: Thanks for all the help.

:Fixed the volume, it was ground problem.

:MP3 Player adapter working, volume controlled by radio.
:The only problem I have now is when I remove the MP3 player plug from the jack, radio plays at about half volume turned all the way down. When I turn it to increase volume, it take about half a turn then it gets louder as I turn it. I used shielded wire as instructed.
: Thanks for all the help.

While we wait for CV to pick up the phone, I will suggest what you already figured out. A resistor you added to the jack circuit is feeding the radio signal by bypassing the volume pot. Remove the most likely suspect first. You may want to redesign the circuit, which we can't see.
I have had good success using a transformer coupling circuit with no resistors. I take the transformer from a 12 volt wallwart and feed the MP3 signal to the short winding, long winding to the volume pot. I just solder left and right channels together. Some will say that is wrong. :>)

surprise you didnt take the chance to show off how you did it on one of your ebay show and tell advertisements. i mean 'restorations'

:Yes.
:You can insert a jack on the high side of the vol control between C3 and the vol control.
:The vol will still operate.
:
:Or insert the jack on the wiper of the vol pot where it feeds the grid of the 1st audio.
:Then the Mp3 player will have to be the vol control.
:
: