I finally got a spare 2 hours to try the often cited procedure of re-melting all the solder joints on the cruise control brain box yesterday. It had all the symptoms of this being the problem - erratic/sporadic kicking in of the function, going off as soon as you let go of the lever, bucking and hesitating when it did work, etc.
I'm delighted to report that it now works perfectly! The price of these rebuilt or new is absolutely out of this world, IMO, and this is such an EASY FIX!
A couple of tidbits on the procedure:
- On my 380SE, the bolt that holds the control box bracket to the brake pedal assembly is easiest to take out with a 2" extension and ratchet with a 10mm socket to loosten, then use a palm wrench to turn it out of the bracket.
- Goof Off works well to remove the gummage from the little piece of foam strip stuck on the circuit board, as it doesn't eat the printed circuit board coating.
- Use a magnifier light to work under, and frequently dip the iron tip in flux paste then wipe to keep it clean.
I was wondering how many others had done this job too, and the success rate?

Good threads for reference:
Yes, we have cruise..
Cruise Control Repair Articles

Finally found: Article on adapting the stock MB cruise control stalk to an aftermarket kit. Here's the link to the thread, which has a handrawn wiring diagram, about 3/4 way down! http://www.benzworld.org/forums/forums/thread-view.asp?tid=1228742 This is the kit that he is connecting to: http://store.summitracing.com/default.asp?target=egnsearch.asp&N=400615+314455&autoview=sku

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1986 560SL
2002 Toyota Camry
1993 Lexus