Well for starters, I'd disconnet the speaker wires from amp. and get a multimeter and ohm them. There should be from 3-6 ohms per speaker, varies abit on what type you got. If this is not the case, you might have an amp. built into you car like
I had.

Also, be sure to check that you have wired the + terminals from the amp. to the + terminal on the speakers, and that they are all wired the same.

These are my first two guesses.

TommyE