For some reason, all I'm getting out of my rear 6x9'* is bass. These are aftermarket and were installed by the previous owner; some Sony Xplod (crap, I know). They sound alright on the front doors, so I'm not complaining. Are the rear 6x9'* supposed to play stereo sound? Or just bass...

sound too, check the wiring to see if they are hooked up right, then see if they are 2 way if the tweeter on the top is still wired to the speaker

Your tweeters are probably blown

That could be it. I'll test some other speakers on there and see if they work.

I have a pair of diamond audio 6x9'* in my rear deck and the one on my drivers side has a messed up tweeter and I hate the way it sounds! This is my third pair in 2 years! I guess this tells me something. Doesnt it?... I am probably gonna put focal polyglass 6x9'* in their place.