Hans, I had the same symptoms, my switch on the column was bad. Make sure you have German wattage bulbs in the stop lights, the wrong bulbs can interfere with cruise control operation . Then, here is basically how to check the cruise:

(B)Check actuator(/B)
* ohms on actuator 2-pole should be 10-22, higher or lower- replace. remove 2-pole coupling to check.
* check vacuum and vent line on actuator for leaks or improper attachment.
* hook vacuum line going to the actuator up to a vacuum gauge, start engine, rev engine, vacuum should NOT drop to zero, if it does check vacuum system. turn off engine.
* connect vacuum pump to actuator(remove vacuum line to actuator). connect actuator to battery(remove 2-pole coupler,hook one pin to neg. and the other to pos.),pump vacuum pump, cruise control linkage should be moving smoothly to full throttle, and should hold vacuum. If it does not, check linkage for easy operation, or replace actuator. Disconnect actuator from battery, control linkage should move to idle position, and vacuum should remain constant, if not replace actuator. Put all the connections back together.

(B)Check Switch(/B)
*check brake light for function.
*pull off 12-pole coupling from control unit(remove panel below steering wheel to access control panel)
*switch on ignition and complete these tests on switch coupling with a voltmeter:

neg.to 12 and pos. to 8/ switch in REST position/value on voltmeter >11v (if you get zero reading on this step
check your stop light switch and connections to it)

neg.to 12 and pos. to 8/ switch in OFF position/ value on voltmeter 0v

neg. to 12 and pos. to 5/ switch in REST position/value on voltmeter >11v

neg. to 12 and pos. to 6/actuate BRAKE/ value on voltmeter >11v

neg. to 12 and pos. to 9/ switch in ACCELERATE position/ value on voltmeter > 11v

neg. to 12 and pos. to 4/ switch in DECELERATE position/ value on voltmeter > 11v

neg. to 12 and pos. to 10/ switch in RESUME position/ value on voltmeter > 11v

If you do not get the above readings, you probably have a bad switch.

(B)Check sensor on back of speedometer(/B)
*Your version should be the second version which is mounted to the back of the speedometer. with an ohmmeter you should get a reading of 650-1370 ohms. higher or lower replace sensor.(anyone with the first version which was not mounted on the back of the speedometer should get a reading of 50-106 ohms.)

If your cable controls are alright, and all of the above tests are okay, then you need a new control unit. That is what most people say goes bad, but I do not believe they are testable, at least not by the home mechanic.

I hope this helps.