Sounds to me like you have hooked the rear speakers up out of phase which means that the positive and negative leads of the speakers are reversed on one speaker from the other. What this produces is a weird mono effect with no bass at all. Make sure the positive lead in the car hooks up to the positive terminal of each speaker. Likewise at the stereo to make sure the polarity for each speaker is correct. To check for correct polarity on a set of speakers rotate the balance control left to right and you will have bass at the extreme left or right but none in the middle..Hope this helps.
