Pull the knob off, remove the big nut, and the switch will come out after you twist it counter-clockwise out of the slot. You will see what I'm talking about when you get there. It is intuitive. The canon plug pulls off the back. It should only take a few minutes to change the switch. I think you will have to take a few screws out from under the dash to get it to drop down far enough to slip the old switch out and the new one in.

My switch did the same thing. It turned out that all the ceramic inside had finally self destructed.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles