the servo tower switches vacuum to the hot water valve on the firewall and all the actuator pods under the dash, just look at the mess of vac lines which come into and out of it at the rectangular shaped plug.
When the motor starts sticking it will wind up in some combination that is either good for winter or good for summer. Worst case is it leaks fluid and your engine overheats
I presently have a stuck servo in my '79 300TD and consider myself lucky to have found a rebuilt one with low miles on it on eBay. Mine current servo is around 12 - 13 years old since I bought it for around $300 as a fresh rebuild so I figure I got my money's worth.
These things are the Achilles heel of the pre-monovalve Mercedes.
Put your mityvac on the hot water valve to actuate it open, apply 12V by jumper cable directly to the circulation pump (observe proper polarity) then drive the car and see what you get. If you don't have heat or enough of it its the Servo, as it has a valve at the bottom that allows coolant to flow into the vacuum operated water valve on the firewall. Its the motor in the Regulator/Servo that is probably goofed up and not working.