On a 190, the aux pump will be on the firewall. But its unlikely to be the problem. When the pump fails, it shorts the system and trips the circuit breaker. Its remotely possible that its failing intermittently, the way to check is to just unplug it. Its not critical to the operation of the heater.

The 190 system is enormously complex. Temperature is set by the head unit, but there are half a dozen hard to reach components involved. First there os a sensor near the mirror. There is a tiny fan connected to this via a hose as described. But again, the fan and hose could be discarded and ot would have noimpact on your particular problem.

Behind the dash, just above the passengers knee, there is a five port switchover valve. This reacts to various signals from sensors and the head unit to apply or vent vacuum to any if five vacuum pods. Mercedes made a test box for dealers to verify its operation. Without that, testing is a complicated process of applying various electrical signals and verifying vacuum operation with a Mityvac.

There are five vacuun pods in the heater circuit. The two most relevant are the water valve located behind the battery, and the air blend flap located at the very top of the heater box (requires pulling the entire dashboard). There is also a blend flap rheostat next to the vacuum control that needs to be tested.

Theres a fairly good writeup on this system online. Google "190 Cacavas"