I recently bought a 2007 Ford Edge SE, with 4 factory installed speakers and a basic AM/FM/CD radio. The rear speakers are not working. I switched them with the front speakers and they worked, and I attached a 9volt battery to the harness and sent a signal to the rear speakers and that worked. I swapped out the radio with a used unit to no avail. When I press 3 and 6 on the presets to run a self test, it only tests the front speakers, leading me to think there is a setting in the radio that I can change to allow it to recognize the rear speakers. There is supposed to be a fade control, but it is not in the menu, leading me to think it is the receiver, but again, i have swapped that out. Any ideas?