In my case, I am using the radio from my '66 Ford truck and using the input to listen to music from an mp3 player. The radio is about as simple as it can get: AM only, mono output but this can be done for stereo just as easily (more easily in fact!).
Here is a quick video of it in action:
The parts count is fairly low:
any RCA jack with 2 NC shunts: (sourced locally at All-Electronics but available online here for $0.50)
Hookup wire: (I had this on-hand; about 2-3 feet is plenty)
Optional 80-220 Ohm resistor (this is only necessary if your stereo is mono)
Optional enclosure to hold the jack
Soldering iron and solder
Step 1: Fix (Optional)
Doing my diligent google research, I was unable to find any schematics online and only the tiniest bit of background information on my radio at all. Luckily, you do not really need schematics to do this hack (but it could make things simpler). This site is a decent resource for information on caps and radios for those that are interested.
In general, the simplest first attempt is to replace all electrolytic and paper capacitors. These capacitors almost always go bad in old radios because, over time, they dry out and their values stray. Replacing them is easy and cheap enough so this can be done very quickly.
In my radio, there are no paper capacitors and only two electrolytics. I used google docs to make an extremely crappy picture with their positions (electrolytics are polarized don't forget!) and their values. I replaced the caps, did a few other minor things, and it worked! The power cap in the picture was not replaced.
Here is a little video of it functioning: