speaker polarity

Do a series of tests by switching the speaker wires at the amplifier.  Using a test tone will help you hear the difference in db better than playing normal music or you could try both.  Manipulate the RCA inputs (or balance and fade on the deck) so that you can test one set of speakers at a time, left compared to right, and fronts compared to rears.  The output will be much louder when a set of speakers are in phase.  It's easiest to tell by using bass tones, something all the speakers can play, like about 60 - 80 Hz. 

Make a chart before you start with the steps of the testing procedure, and include every detail such as "switch balance to the left, fade to the rear".  As you test, check off each step, and this will go a lot smoother.  This method is time-consuming, but when you don't have to disassemble the car it can save a lot of time.

You can alternately use a DMM to test AC voltage output at the amp speaker terminals.  The reading will be a positive number if the leads are on like polarity, or it will read a negative number if you have positive lead on a negative terminal.  This is more confusing, though, because of the way bridgeable amplifiers work (see about halfway down the page  HERE).  It's difficult to be sure which terminal is supposed to be positive, and the meter reading might also be difficult to decipher.  The listening test is the easiest to do.

A speaker will make a noise no matter the polarity of the battery.