Here's one that stumped me and my father in-law for several weeks. It's licked now.
My Saab has AM and FM receiving antenna built into the rear window. For the car's FM (or was it AM?) radio, the antenna wires look just like defroster wires. For AM (or is it FM?) the defroster wires double as an antenna, with a choke feeding the ground side of the heater coils.
The problem I had was that these wires provided a perfect route into the car for my transmitted RF. The solution was to put a big split-choke on the DC power lead which feeds the rear window defroster.
More details in the elmers forum here: http://www.eham.net/forums/Elmers/115677
So - moral of the story:
If your car uses the rear window defroster wires as an antenna and you are getting lots of RF into the car and can't figure out why, choke the defroster power line.
See you mobile