Mine is doing the same thing. Do a search under "cruise control amplifier" or "cruise control amp." - will give you lots of info.
BTW - I found out the hard way about using the cruise in heavy rain - totalled a brand new (three month old) S600 sedan because I hydroplaned. Needless to say the insurance company dropped my parents because of that one. I was very lucky to have walked away from it with only minor cuts, bruises, and a couple of broken ribs.