Why would all the rear speakers not work after installing a new deck when they all worked before and now only the front two speakers are working?

Answer 1

also, if you have a premium sound amplifier it could cause a bit of trouble. I JUST figured out that my 1997 harness doesn't power on my aftermarket radio. I could hear feed back through the speakers but no power/sound. after rewiring to ignition radio turns on and NOW allthe speakers are working perfectly.

AnswerI willstate the obvious first and you probably did this but I have to ask. The connections on the speakers are still on the speakers right?

I shall assume so and move ahead. I will assume the stereo wiring is not accessable yet so check these things first. Check to ensure the stereo is set to play sounds to all speakers. Then try both CD and radio during the next tests. Start the CD playing and then turn the fade and balance controls. Do you get a left to right change, can you fade out the front speakers when you turn the selector to play in the rear. Then the fronts come back up as you fade to the front. From the question you asked I would guess the answer is that the other speakers work OK during this test but the rear speakers don't. If the connections were ok on the speakers then the only other culpret is that the wires got pinched when the decking got replaced. You can follow the wires to find the pinched wire or you can run a new one.Before I did that I would gain access to the rear of the stero and find the rear speaker connectors. Then with a new wire laid over the seats and connected to the speaker I would ensure the speaker has sound. If so then replace the wire. If not then you could have a bad speaker or a blown amp that feeds the rear speakers. I have seen both happen. Just depends on the brand and model you have in the vehicle.

The simple answer to the question you asked,, You have a bad wire, a bad speaker or a blown stereo. Since both are bad I would bet someone disconnected something when working on the deck and you just need to find where they did it.