I used an Audiovox CC100 aftermarket cruise system on my '82 300SD for several years, it was more than strong enough to hold the throttle and keep the car around 80mph easily. It had an electronically controlled vacuum pod with a cable that I attached up on top of the valve cover to the linkage, worked great! I even took the old cruise amp out, cut it up and soldered some of the control wires to the socket so I could use the stock stalk. I had set/increase/decrease/cancel + brake pedal would also cancel it.

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