This isnt really an explorer question, but I believe the Mach audio system between explorer/expedition is the same. Besides, how could I not ask the extremely helpful and knowledgeable member base here before I go join another lesser forum?? I am installing an aftermarket amplifier/subwoofer into a 1999 ford expedition with mach audio. Through experimenting, I discovered that the head unit will not send the low (below 150 hz) signal to the rear of the truck without the factory amp plugged in. The pins on the amp connector include: power, ground, sub +/-, and high audio mute. I suspect the amp grounds or provides power to this "high audio mute" wire to tell the HU it's plugged in. I don't want to fry the head unit trying to ground this wire, I'd like to be sure. All I need is a low(hz) signal through the stock sub+/- wires to signal the new amplifier, as opposed to getting the signal off one of the rear speakers, which would only give me a filtered signal (high pass). Does anyone have any insight on this? A very specific problem, and a very specific situation, so ive found no use in searching. Help is much appreciated!