Walt, this approach assumes that the cruise control amp is faulty, and not the vacuum servo.
The 10 pin analog cruise amp (MB diesels to about 1981, I think) has three very common failure causes: 1] The LM-2901 comparator IC chip. 2] The dual gate N-channel FET. 3] Failed solder joints / broken PC traces. 4] Some odd combination of all of the above. The above parts are about three bucks at Radio Shack, but are probably cheaper elsewhere.

Pull the amp out to a clean bench, and take a look for failed solder connections or cracks. Retouch suspect ones. While you, or your electronic savvy helper, are in there replace the two active components I mentioned. I repaired my amp, and those of four local friends with this method, and we're all cruise control positive. Another required a few days, and some rather expensive test equipment I have at hand. If you draw the short straw, and get stuck with that kind of failure, then an exchange box is your best bet. I think they run around $200. Inquire first with our hosts at Partshop, since they're such good sports.

Good luck,

Luke - An engineer who drives to work everyday in his '78 300D