Do you have any thick lead panels to play with? Perhaps one of those bibs they use at the dentist when taking X-rays?

If it is something like was just discussed, I would think that you should be able to affect the sound by moving the tuner around. If you wanted to be extremely diligent, you could also hook up an extension lead and move the tuner very far away (as a test of course) - or out of the car.

Bruno