you could try the search feature in this site, but there is an other equally good site where I have seen this topic recently. Go to
www.mbnz.org I have seen some recent posting of people upgrading their systems with MB quarts units ($500 for 4 speakers), but I think you need an amp to take full advantange of those speakers.
To take the side panels off you should firts get some of the clips that hold the panels to the door (they will most likely brake). Somehow I managed not to brake any.
I have only taken the front ones off and for that I just took a small flat head screwdriver to lift the simulated plastic covers from the door handles (not the door latch). Inside you will find 1 or 2 phillips screws which you will have to take out. Then remove the triangular plastic trim from the door, the one that is close to the windshield pillars (it will come out by pulling on the sides). With that piece out of the way you can slide your hand between the door panel and the door ( on the side not the top), and slowly pop one clip at a time. You may want to use thick leather gloves if it hurts to do this. Onece all the clips are gone, lift the panel up and slightly pull apart from door; you will need to hold it in this position while you disconnect the latch mechanism from the door (hard to explain but easy). I imagen the rear doors can be done in a similar way.
hope this helps...