I ran a 1/8" cable to a stereo 1/8" splitter. Ran one side of the spliiter to an FM modulator (much better than an FM transmitter as there is NO interference) wired between my radio and antenna. Then the other side of the splitter went to a 1/8" to RCA cable to my amp.
I keep the volume on my radio at full and use the MP3 player for volume control.
http://www.crutchfield.com/S-9oDEWuNVIo6...