We are going to modify an old boom box (AM/FM/CD/Tape) to add an aux in cable so we can connect an iPod or phone to it. I'm using a Koss HG835 boom box that I found at a thrift store for $15. When we're done, it will be able to play from the aux in cable, CD, and radio.
Some parts of these instructions are specifically for the Koss HG385, but the procedure will work on pretty much any boom box that has a cassette deck as long as you can find the pre-amp chip.
There are 6 screws on the back that attach the front panel to the back of the boom box. I needed a long 8" screwdriver to get to them since the screws were recessed so deep in the hole. Make sure the stereo is unplugged before you start working on it. When you pull it apart, you can disconnect some of the ribbon cables plugged in to various places so that you have more room to work.