The Cruise control shuts off when you tap on the brakes, and the way it knows this is when the brake light comes on.

Presumeably, if the resistence of the specific brake light bulb is not what the Cruise control expects, then it won't work.

I don't think this could logically cause the sort of problem you are having. I am not an electrician, so please don't take this as gospel.

Now that you say that the speeedo nedle bounces even with the Cruise Control off, it seems more possible that the speedometer cable or one of its connectors is at fault. A new speedo cable is sure to cost less than an overhaul of the CC.

Good luck getting this to work.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)