Stumped 96 Taurus

Hello everyone...I have a problem that I can not seem to fix.  Any help would be greatly appreciated!!  I have a 96 Ford Taurus.  I have a pair of Boss 10's and a 700w Boss amp and I am running the amp off  of my two rear speakers for signal.  Here is my problem.  I have about half power coming out of the amp because if I turn it on up the speakers will begin to hum a constant note.

So I have left the gain settings alone and I can listen to it fairly loud. This is the part that really gets me though. Whenever I turn the car off that loud hum starts playing and It will not stop until I a door!!! Also when I was trying to tune the system from the trunk I noticed that I could turn the gain up when the trunk was open but as soon as I would lower the trunk about half way closed the hum would return and I would have to lower the gain again.  I replaced the wiring and the amp and I still have the same problem. Could it be where I have my wires ran??  I am sure you all can understand how this one makes me scratch my head!!!  Thanks!!

Thanks for the quick reply....My ground wire is about one foot long.  The gauge is 10.  I know that the gauge is probably too small but I had this same problem when I had my smaller 300 watt amp.  I just went out cleaned the ground connection and re-attached it.  Thanks again