If the interference is being picked up from the wiring, try this: Speaker wires: Short both( and -) speaker wires to a chassis ground with .01-.03 microfarad disk capacitors. The polarity of these capacitors(Available at Radio Shack) does not matter. If that doesn't work, then try a filter capacitor on the Power supply to the radio. Radio Shack has these filters, and the disk cap. for the speaker wires. It won't hurt audio quality or anything like that. Good Luck!

 

RF noise, static make it impossible to listen to the AM frequency on the radio
in my 1998 F-150. The dealer has replaced the radio and checked the grounding
of related electrical components. The noise is induced only when the engine is
running. The noise gets louder when the lights are on. Any hints on whats causing
this proplem.

 

I would be very suspicious of the Alternator. There could be a problem with it or the voltage regulator in it.