check the ribbon cable that connects the units to the amp. I would assume that if the cd player is reading the disc, and there is signal present, the ribbon cable is suspect. If you can, get switch cleaner, or isoproyl alcohol and clean the contacts on the ribbon cable and the connector on both ends(amp and cd). If this doesnt work, then I'd atempt to get your hands on a multimeter and check the continuity and resistance of each lead in the cable.
afourtrackmind, April 2008