If your temp guage is between 80 and 90 you should be getting warm air. Not sure about 1980 model but my 90 starts pumping warm air as soon as the guage starts moving. If I set my fan to auto setting the fan won't blow until the coolant reaches a temp higher than interior temp.
Vacuum/clean the interior temp sensor. (long shot but easy to do)
Look into mono valve....If you didn't get any heat I would suspect this but you say you eventually got heat????

I wouldn't cover the radiator as your running temp is fine.

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82 300D....went to MB heaven
90 350 SDL....excercising con rods