Check that the glass seals firmly against the side pillar.On my convertible, the front driver's glass went up too high, which created a gap & water leaked in behind the drivers' seat.

__________________
87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)