Check your rear speakers to see if the magnet has fallen off of the tweeter portion of the speaker. That is common on the speakers in the 850's. I just fixed the rear deck speakers in my '95 850 sedan. These small magnets had fallen off the the back of the tweeter (or midrange) speaker insert down onto the main speaker cone. I reattached the magnets with a gel epoxy. Both rear deck speakers had this same condition and now they sound like they used to! Also, my radio has the "premium sound system" logo and I do not have an amplifier.