The AM side is completely dead? Or just that it seems it's next to impossible to tune in one AM station without any cross talk from near by stations?
Look at the ant ground on the frame, try doing a variation of the INFLREST treatment and check the reception. If still no good I'd do a call to Dr. Don.
If the AM band is just dead, static or complete silence then I'm thinking CDM box or perhaps a bent pin on the connector at the back of the head unit, or the selector switch could have broken in the FM position.
