Rear speakers

I've got a 1999 pontiac gp GT fully loaded with the bose system and the rear speakers only play bass. They thump but you can't hear a guitar or anyone singing. I believe this problem is common amoung gp's but does anyone know how to fix it? :sly:

i think there is some sort of frequency cap connected to the rear speakers or something. either that or a crossover somewhere

I don't know that much at all about speakers. (i.e.) Whats a frequency cap look like and what would I do to it and what's a crossover and what would I do to it? If you turn the fade all to the back all you can hear is the thumping of the bass, I'm pretty sure it used to play the music not just the bass