The VDO cruise control, and there are several types that MB used, does have some "weak points" as most of vintage owners know. From around 1976 to 1980, the system was vaccum activated with a 10 pin amp, while from 1981 to 1993 it was electronic with a 14 pin amp. Since 1993, the cruise control has been integrated with the "engine management" computer and by removing the additional amplifier, made it more reliable.

Here's a link to a nice comparison article:

http://gdl-online.com/differences.html