diagnostics:

https://sites.google.com/site/mbcruisecontrolrepair/home/diagnostics/1982-1995-electric-servo-cruise-control-diagnostics

repair of a cruise amp (half way through the first post is your model):

http://www.peachparts.com/shopforum/tech-help/313824-how-cruise-control-amp-repair-14-pin.html

same guy can also do it for you. his new website:

Cruise Control Repair