Is should be possible to put a jack on a old car radio to use a I-pod input. Has anyone tried this? I would think you could go in on the volume control, just put a switch to go from radio to aux input, I know a lot of old car people that I fix am radios for, and they would love to run their I-pod thru their old am radio. What does anyone out there think?

Yes, this has been done, and no, a switch is not really necessary. You can purchase an audio input jack that has the switch built in- plugging into the jack disables the radio and switches in the external audio source. Radio Shack or eBay have these jacks.

Since IPods have stereo output, you will need a simple resistor "mixer" to combine the two channels into one for the radio.

Here what you need:

:Here what you need: I used the very same circuit, but with 2, 10K ohm resistors, would 1K or 2 K be a better choice? My circuit seem to work ok but I wonder if it has too much attinuation

The radio has very high input impedance and the IPod will have low output impedance, so the value of the "mixer resistors" is somewhat irrelevant. Anything in the 1K-20K range will probably work fine, with "low input" being easily compensated by the IPod's volume control setting. Generally speaking, however, the higher the value of the mixer resistors, the better- since the higher resistances put less stress on the IPod's output stage.