I bought a car recently. It runs like a dream, but the stereo's archaic -- no audio input for a phone, just tapes and CDs. While you can buy cheap tape-to-AUX converters, something about the coupling mechanism adds white noise to the signal. Instead of paying a bunch of money for a new stereo system, I cracked the sucker open and directly soldered in the connections.
Step 1: Lay down ten bucks for stereo removal keys and yank that machine out.
Step 2: Open her up.
Step 3: Find the pins to the CD player's ribbon cable output. Identify RCH (right channel), LCH (left channel), and find where ground is. In my case it wasn't labeled but I traced it to the pin between the left and right channel outputs to the ribbon cable.
From here, the CD player sends the audio signal to the main amplifier. This audio signal can be replaced with the audio output from a phone or mp3 player!
Step 4: Cut the three wires of the ribbon cable, solder in some of your own and extend them somewhere accessible.
Step 5: Carefully solder these leads to the male end of a male/female audio jack converter:
Step 6: Your stereo's now updated to the 21st century! Plug it in and blast some metal.
Still working on making it look prettier. Another improvement would be to use a bluetooth audio receiver module like this one, which can be completely encased in the stereo unit and accessed remotely. For the one linked to, the powering method is via USB (5V) -- so you'd need a buck converter to lower the car's native 12V DC or you could find a 5V line in the amplifier board (there's bound to be one somewhere).