1997 ford taurus stereo

Hello all.... I have came across a problem that I have never seen before today and I was wondering if any of you have seen this before???   We reciently put a stereo in a 1997 ford taurus and everything was working fine with the high level inputs into the amp for signal in and the remote running off the radio fuse.  Now this is where the weird part comes in.   The guy came in today and his stereo will only play in park and neutral.  When he is in any of the other gears there is no output to any of the speakers or anything, however there is still power to his stock headunit, just no signal.  Has anyone ever came across this before if so please help!!!!   I just want to clairify that when his car is in park or neutral everything works perfectly.   I dont know where to do from here so any help would be appreciated.  Thankyou all in advance. 

It sounds like the factory amp turn on lead is losing power.  Verify this with a multimeter.  The amp is on the driver's side of the trunk.  Ford uses a 5 volt turn on, so if you want to feed this wire accessory power, use a 1.5 k ohm resistor inline.My ex once told me I have a perfect face for radio.