I've bought 3 from this guy:
http://www.benz-store.com/cruise_control_amplifiers.html
$155 exchange. I've also bought 2 heater fans from him - $105.
The CC amp is a silver box located just above your knee under the knee panel. 90% of the time it's the amp, though I have also replaced one actuator which his mounted on the engine and controls the throttle.
I've never had success soldering them, though I'm pretty good at it.
The Mercedes CC works very well - I use mine 2 hours a day and love it.
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1984 300Sd 210k
Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer