(This will likely be moved to the audio forum)
Check this thread for some ideas.
http://www.peachparts.com/shopforum/showthread.php?t=201422
Note too that the speakers in the dash are 8ohms, if original, and when you connect a modern unit designed for 4ohm speakers, the volume will be about half the potential and the sound quality will be bad. You'll need to change these speakers at the same time. No biggie, but just a heads up.