With the engine at operating temps, and running, Carefully grab the heater hoses to feel how hot they are. They should be pretty warm to the touch. If they are not, then you are not getting any coolant flow to the heater. Causes for that could be a blocked heater hose --did the previous owner block off the heater core for some reason?--or low coolant level. If the coolant level is low, there will be reduced or no flow to the heater.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags