I believe you can do it two diffrent ways:
1. Use a head unit with an auxillary input. Connect this input to the output of of a high quality sound card plugged into the motherboard's PCI slot. For AM/FM, just use the HU, for MP#'s use the AUX input.
2. Don't use a head unit, Use a High-quality sound card, and a TV/FM tuner PCI cards. Connect the output of the soundcards via a stereo mini-plug to RCA adaptor to your amps.