RADIO problem while engine running

1990 300e is great, my only problem is the radio. It is strong while the car is off or on ACC, but start the engine and the volume goes away, leaving very few stations with signal strong enough to overcome the background static and noise. The radio gets voltage from different fuse paths when the engine is running and not, so I think that that is where I need to start looking. If anyone else has experienced similar problems, please respond and help me. tx, paul