The back seat has to come off - a tab at either end of the rear seat bottom, slide outwards then pull seat bottom forward and up; 3 8mm head screws along the bottom edge of the seat back, then lift seat back off hangers and pull forward (it takes a heck of a lift); pull speaker grilles forward then up (be very judicious of how you apply preasure because 13 years of baking in the sun has made them brittle.

The main speakers are the 6.5" variety but the factory speakers are molded into an angled cylinder base. Some folks cut off the old speakers and glue on new speakers. A 90 might have separate tweeters.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD