Got something W/ a headphone jack? play it in your stang for $100

I like the idea of leaving my Factory radio in place, but I'd like to be able to pump both the radio and the MP3 through the amp. I can't believe someone doesn't make an amp that will take a regular radio speaker output as an input.
There are, you just have to look for it by reading the specs. If it does not accept speaker level inputs, you can run the speaker outs to a line output converter, which converts the speaker levels to something an amp can use.

If you must keep your factory radio, then an FM transmitter for your mp3 player might be of use to you.
 
Great! thanks for the tip. Yes, I must keep my factory radio. I thought about the FM transmitter set up, but the idea of hard wiring it to an amp seemed like it would make for better sound quality. An FM xmitter is probably a better solution though, given that the road noise is so loud that any loss of fidelity probably wouldn't be noticed.