No heating Mercedes G 250GD 89

Assuming your 250GD is of a W460 chassis, this is a common problem from summer to winter settings of heater. During summer the heater valve temperature would be usually set to cold; that setting for months bonds the rubber seal to its seat and when the temperature valve is set to hot, the seal does not lift off its seat giving cold air when hot should be filtering through. 

It is better come summer or winter rather than to fully set the valve settings to fully hot or cold that you come back a little so the rubber seal inside the valve does not fully sit on to its seat hence availing to seal bondage.

Check the feed pipe from the engine into the bulkhead  to the heater matrix if it is hot, that will make ensure you have flow of heat to that point. If there is no heat sensed after the heater valve, it is the vale that is blocked or the seal bonded on its seat. You may try to work the valve open by opening and shutting it, or replace the valve. If heat flows past the heater valve; you could be looking at a blocked heater matrix. 

By undoing the inlet and out let hoses to the the heater matrix, you could try and blow pressurised air from the outlet of the matrix to see if the pressure will lift the suspect seal off its seat ( heater valve in the hot air position).