P38 Radio Signal Problem

Hi, wondered if anyone could help. My radio signal works perfectly until the ignition is turned on, I then lose 90% of my signal. The radio doesn't lose power, just the signal. I have test for power to the 2 amps in the rear of the car and all is well. I am not sure if any of the amps are faulty, not sure how I can test them, I have taken the rear left "FM only" amp out of the circuit and it makes no difference at all, and I have powered off the other amp and I noticed a slight drop in signal. I personally do not think it is an amp problem as it works fine with the ignition off, but some kind of suppression problem?? any help would be very useful because its driving me nuts!