Rear speakers only giving off faint treble
 

Before I start, keep in mind I have a 2001 Oldsmobile Intrigue. That being said, it's still GM, and the wiring (from what i've seen) is generally the same as a newer-gen Bonneville.

My car has 6 stock speakers. 1 tweeter per door (front), one 5.5" per door (front), and 2 6x9's on the rear deck. I am running an aftermarket Pioneer head unit, using a wiring adapter.

Now, my problem is this: my tweeters and front 5.5" speakers sound perfect. my rear 6x9's, on the other hand, are only making a TINY amount of noise, barely audible. When you put your head near them, you can hear some of the high notes of the song (absolutely no bass), but nothing else. Even with the fade set all the way to the front, that's all I'm getting.
I've checked and re-check my wiring; everything seems to be done correctly.

Is it possible that a factory crossover is somehow messing with the signal being sent to them? I know that there has to be a crossover to deal with the tweeters, but I cant imagine how that would affect the rears. I have no "loose" wires, only the illumination+dimmer+amp turn-on wires, which have nothing to connect to.

Thanks guys.

blown speakers

I forgot to add: the car was previously owned by an old lady, and she never used the stereo. When I bought the car, I immediately switched to the aftermarket head unit, and they never once worked for me. So they could've just come from the factory as "lemons", or somehow I blew them while wiring my new deck?