LSWon00

I'm trying to install a flip-up deck into a friend's 2003 GT with the regular MACH stereo. I have the harness that connects wires to power the head unit and all works fine. The problem is, is that the volume is very low and the front door speakers do not work. I assume that is what the other factory harness is for, however I can find no adapters to use to power the front speakers. If anyone can help please do.
~Brad