You don't have Bose, correct? The fronts are driven off the head unit, the rears and sub are driven off a small amp that's inside the sub enclosure.
I'm afraid I don't have much advice, except it's probably an issue with the amp. If you want to check the connection, it's behind the panel on the left side in the hatch. There's a large plug that goes into the top of the sub enclosure
Good luck! :thumbup: