I have an Alpine HU in my 94 Mazda B3000. It's manual. I'm getting interference on am radio stations but haven't noticed anything with fm. Sometimes the interference oscillates with the engine rpms. Any suggestions on how to trouble shoot this? thanks!
I would suggest to bring in a portable am radio and see if you listen engine noise through it placing it near the antenna, if you do the problem lies in the electrical system emitting RFI. If after servicing properly the electrical it doesn't go away a better quality antenna, antenna line rerouting or even antenna relocation can be considered.
If disconnecting the antenna from the HU results in the noise persisting you can try to ground your HU chassis to the chassis of the car through a connection. Also check all grouding and add a power lead noise filter to the HU can be tried.