1992 mercedes 300E will not blow heat,fan runs?

The heater control valve, and this car also has an auxiliary coolant pump for the heater, is probably the culprit. The good dews is the unit is rebuildable as well as a fairly inexpensive part if you choose to just replace it. It is unlikely the car has a bad thermostat if it is warming up and not overheating. It should have a fail safe lock-open thermostat, but that doesn't mean the previous owner kept to the script and replace it as such. If the coolant pump has failed you're looking at a bit more money for the fix. Lastly there is the remote possibility your W-140 has a wiring failure behind the heater/AC control panel. These cars had wiring problems and they come forth as they age. Best of luck.

Is the engine temperature gauge showing heat? If not, check if the thermostat is stuck open. Otherwise, check that the hose(s) to and from the heater core are not kinked or blocked. Also check the mono valve ( it controls the flow of coolant into the heater core ).