I replaced the 6x9 rear speakers in my daughter's Grand Prix with some Pioneer aftermarket coax's. The speakers (admittedly low budget) did not have polarity markings on the lugs, nor was the factory wiring marked. Installation instructions made no mention of +/- wires. Now, with the L/R balance setting in the middle, it sound like there is no bass at all coming from the rear. If I move the balance all the way left or right, the bass comes back and the speaker that is on (L or R) sounds great. Could incorrect polarity be causing this? How do I know which speaker is wired wrong?