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:
Step 2: Background Information
The first point of order is to understand your radio. If there are schematics available for your radio get them! It will take a lot of the guesswork out of this.
I was unable to find schematics for my radio but it is simple enough to generalize. Older radios are extremely simple and mine is no exception; it is an AM mono radio.
The key component, as far as auxiliary input is concerned, is the pot. The pot is typically broken up into three distinct functions:
2) Volume Control
3) Tone Adjustment
The middle of the two which governs the volume (see the closeup, picture #3 in this step). There are three prongs coming out of the pot. The middle connection, called the wiper, is wired directly to the amplification stage. This wire will be left untouched. The remaining two prongs are connected to the source (antenna in this case) and ground.
To be able to tell which prong is connected to which, position the radio so that the knob is pointed towards you and the prongs are pointed downwards towards the floor. In this situation, turning the knob clockwise increases the volume which means that the source is connected to the right prong (B in the picture).
Last is to map out the pins on the RCA jack. Using a multimeter, I mapped out all pins on the jack in the two situations where an external source is plugged in and when it is not (see last picture in the set).
Step 3: Cut, Solder, and Done!
The modification that is required is really simple and can be quickly done. All that is being done is to add the RCA jack in between the source and the potentiometer. Using the pin numbering scheme on the previous step:
Disconnect the wire that is currently the source on your potentiometer. This will now connect directly to the RCA jack.
Pin 1 will go to the ground leg of the potentiometer
Pin 2 will go through a resistor to the source leg of the potentiometer. The resistor value can be anywhere between 80 to 220 Ohms and will allow you to play a stereo source on a mono radio without loss of sound.
Pin 3 will go to the tuner (it will be the wire that was previously connected to the tuner)
Pin 4 will be unconnected if you have a mono radio like I do.
Pin 5 will go directly to the source leg of the potentiometer.
Slap it all in some sort of enclosure and you are done!
Notice, I cheated a bit because my pot is mounted directly onto a greenboard. I wanted to attempt this without making too many changes to my existing setup in case it did not work so I ran the RCA jack as a source in parallel with the tuner. The end result is that it functions but could end up having some problems with running two sources like this.
Now that I know it works, however, I plan on removing the pot from the greenboard and adding more wires to hook this all up properly. I'll update once that is completed!
Much thanks goes out to AMCForums where this was originally posted by member nali. Even more thanks goes out to forum member gryzynx for the extremely thorough explanation and helpful replies. The original forum thread can be found in its entirety here.