Attennae work on a large ground plane principle. For instance your car radio actually has a larger ground plane than your house radio. (the house radio runs on the AC chord ground where the whole metal body of the car is grounded to the battery) Anyway, I would look at all the grounding connections. If you think about it, your rear window defroster is nothing but a dead short between positive and negative (or ground). Could explain why the ground does not come into play until the defroster sets up the connection.
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