Water got into my 87 300e under and behind the driver's seat and backed up into the channel under the left side of the rear seat where the wires from the trunk are bundled. Now the pump under the left side (driver's side) of the rear seat makes chugging or clicking sounds; previously it was silent. What does this pump do, and what should I do? Thanks.

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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)

Isn't that the vacuum pump for the locks?

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2012 E350
2006 Callaway SC560

theres a vacuum pump for the central locking and then theres some light controller or something. with the seat removed, the one that makes a mechanical sound when the doors lock is the central locking pump. It should also be enclosed in a type of foam to dampen the sound, and should have helped with the leak...

Actually the supply pump is closer to the passenger side under the rear seat, at any rate, it would be worth it to remove the rear seat cushion (technically referred to as the rear seat squab), the foam rubber covering it and have a look.