Just because you've got a sweet classic car doesn't mean you have to go with 1960s-era technology for your tunes. Hacking your radio for MP3 input is a simple DIY electronics project.
Car enthusiast Nali over at The AMC Forum, an AMC-centric car forum, shares a simple tutorial for wiring in an MP3 input into an old-school car radio. You'll need a few resistors, a headphone jack, some wire, and of course a soldering iron.
When you're done you'll have an audio input on your classic radio that automatically kills the radio when an MP3 or CD player is plugged in and has volume control through the in-car deck instead of the MP3 player. Check out more pics and the schematics at the link below.