ummm...my HU is in the glovebox, and I use it as a preamp for my computer (computer is connected to Auxillery Input)
The best thing to do would be to sacrifice your cd changer and use that for an input. Next best, i guess...would be an amplifier with a giant relay block that switches all 8 wires...that'd probably be about $100 + amplifier. It would be very clunky and inelegant, but it would work. Or you could go with an FM modulator but they sound like *** in a tin can, as someone rightly put it.
I'd say the most realistic option would be ditch the changer.