Problems w/ rear speakers

I just installed a new alpine cd player in my 82 300D, with some infiniy kappa rear speakers. (the orginal size.) the problem is, the rear speakers have no bass to them, although the treble works well. When I bought the car I was told that the rear speakers never had a lot of kick to them no matter what kind of speakers/tape decks that were put in. Is just a defect in how benz built their speaker housings or is there another problem at hand.