From your description of the fault, it may be the fm tuner section. The audio signal from either the am or fm tuner sections will be fed into the audio amp. These days, with surface mount components (unless you know someone who will repair it for free), the cost of repair is likely to be more than the cost of a replacement unit.
One possible (not very elegant) solution is to feed an fm radio signal (audio) into the aux input � but you will need a separate radio and the signal might not be that good with the receiver in the car.