[Tom Goldman]

Too many devices on a common ground can lead to r/f noise .
As was mentioned the fan is the single dirtiest source on the car followed by the electric fuel pump.
I use ferrite core shields on everything .
I use the type with a plastic case that snaps together around the wire . Very inexpensive, available on Amazon ,EBay .
The old school and still one of the best ways to find the source is to use an old cheap AM pocket radio tuned down to lowest end of the band ,around 540 . it will pick the culpret just about every time.