AM/FM is silent. Cassette works but sound is...

Question

The radio sound suddenly stopped. The cassette sound went very dim for a period of months. I paid $35 to H&R Radio in Warminster for repair. They opened the unit and returned it saying that the ''mother board'' was ''corroded'' and therefore the unit was not repairable. Since then some few months have passed and the cassette has recovered on its own to full volume but the quality is somewhat scratchy. I opened the unit and it looks to the untrained eye sparkling clean. It comes from my 1989 Lincoln Town Car which is in near mint condition with just over 100,000 miles and always garage kept. I don't know where the ''corrosion'' factor if true could have come from. Bottom line; I really would like to repair this unit and keep all equipment for the car as original. I've had no luck in finding an exact replacement (Ford Radio E9LF-19B165-AB)