My father's '93 E320 coupe (similar M104 to your car) needed a new engine wiring harness after an engine wash. The wash was obviously the "final straw" for a harness on its way out. Have a look at the insulation on the wiring to the ignition coils near the spark plugs. If it is crumbling then I would suggest the harness is your problem. It will have cracked insulation which the water has got into. Do a search here on "engine wiring harness". A well known problem on many MB models from about '93 to '95. Suppressor boots that fit onto the spark plugs are also known to fail on these engines.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.