I put a new radio/cassette/cd head in my '92 Voyager using a special "amp integration harness" that is supposed to keep the original Infinity factory speakers and amp working but I now only get sound from the front speakers. Is there something I need to do to get the Infinity amp to power the rear speakers? I connected the four speakers leads, + and -, the yellow battery wire and the red AC wire. Did I leave out something?