To the original post: It sounds like you're hearing alot of distortion thru your speakers. Did you burn your own CDR or does the radio also sound that way? The 1st thing I'd do is take it back to the dealership. Maybe they punched a hole in the speaker during installation. Has it always sounded that way, if not then you may have a rear amp IC gone bad inside the stereo. Several things could be wrong, but its hard for any of us to help w/o actually hearing for ourselves.
As for the post about taking a look behind the dash at your stereo, good luck. I tried to connect a simple antenna booster to mine, and had to 1/2 way disassemble my whole dash. 1st from the right side, then from the left, only to find out that the antenna connector was " screwed in" to the back of the stereo. I tried everyway in the world to unscrew it, but just didn't have enuf room....and I thought my Z Car was tight down there!