Audio Systems: FM reception loss, fm reception, signal wire

Question
Whenever the remote wire to the amp is hooked up (and has been with 3 different head units) FM reception goes to practically nothing. Undo the wire and reception is good again. The people at the shop blame it on the amp, but I've had the same set up in a more enclosed area. Any ideas? I listen to the radio more than anything  

Answer
Ok Jimmy, lets look at his logicly, the radio FM works ok when the amp signal wire is dicinnected but cuts out when the amp is powered.
It's NOT the radio
Is it the amp? What powr cable do you have to the amp, it should be heavy enough for the job and fused at the battery terminal.
The next test is to NOT connect the signal lead on the amp but bridge the power connection on the amp to the signal connection thereby switching on the amp, does it still cout out the radio, no then there is a short on the signal cable. if it still does it disconnect the RCA lads (inputs) from the amp and try again, if it is OK then connect the leads one at a time untill the radio cuts out, there is the problem
Peter