Ok so I found a reference of how an MP3 input is added to an older radio. There are two problems to solve. First is the connection into the audio section and second is how to combine the two channel stereo inputs of an MP3 into a mono input because the radio only has mono capability. Here is a link to the HAMB where it is explained. I will look through my material to see if I can find the actual hookup for the truck radio.
MP3 Input / The HAMB 
