96 Marquis LS Poor am radio reception
 

My 1996 Marquis LS has an aftermarket alpine stereo installed.It has a separate power amp/head which runs the stock front & rear speakers.The original amp has been bypassed.The sub has a dedicated amp attached to it.The receiver has a female antenna input,the cable runs to the trunk area and is plugged into an am/fm isolator.This unit has a connecter that is plugged in to a wire from the rear window antenna.My issue is the am reception is poor,fm seems good.When I tune to the am station I prefer,a whistling sound and a blowing across a mic rumble is heard behind the broadcast.This station comes in clear as a bell in all my other cars.My local stereo install shop seems to think emf interference is the cause.I tried disconnecting the antenna cable from the isolator,all reception is then lost.Is there a line filter that would improve the am reception.I tried wrapping the receiver in tinfoil,but no luck.Even a slight improvement might help.The original radio did not have this issue,however this is the way the Marquis came when I bought it.
Any ideas/advice would be appreciated.................-: Thank You