Has the car had a windshield put in? Glass installers not familiar with Mercedes will almost always screw up the rain sensor, assuming they put in a glass compatible with the rain sensor to start with. The overhead control panel is in charge of the rain sensor, though it networks with a few other modules to actually control the wiper. As such, lots of things can be screwing up the wiper operation. The best bet is to pay for a diagnosis at a shop equipped with the DAS tester. This will assuredly be cheaper than throwing parts at it.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff