Check the earths at the rear of the car, there's one below the n/s/r lights, under the plastic panel/carpet, and it could be that the aeriel amplifier earths there. A classic sign of a poor earth at the back of the car is if you have the side lights on and you press the brake pedal, the rear light/brake light goes off, then comes on again when you lift off the pedal, since the side light is finding an earth via the brake light circuit. You may find you get interference also when using the heated rear window, which again is probably earthed at the same point. Using anything else (HRW, brake lights) that shares the same earth as the aeriel amplifier can cause these problems if the earth connection to the body is in poor condition.
Edit: It looks like all the rear lights, wiper motor, fuel gauge share the earth under the n/s/r lights, but the HRW has it's own earth on the C pillar. The radio itself is earthed at the front, but the aeriel amplifier seems to earth via the aeriel itself, I can only see a power connection?
Anyway, check the earths!
Last edited by DaveG; 05/09/2013 08:06. Reason: added more info