In my experience the most common problem is the ground. In this case the odds of all the speakers being wired incorrectly is minimal. Check the head unit wiring (I am assuming you used a aftermarket wiring harness to hook into the factory harness?) red should be battery when key is "on", orange (or yellow) is battery "on all the time", and black is the ground. There may be a orange with white stripe for dash illumination. Blue should be remote turn on output for an amplifier. Even if your negative and positive are backwards to the speakers you should get some sound (however it will sound out of phase) if the ground is good. Also, check any fuses built into the head unit.