I had a problem with my heater not coming on also (must be this Washington weather, huh?), and I found that the heater fuse was bad, although it looked good. What apparently happened was that it wasn't real tight in it's holder, and one end melted where it couldn't be seen. When the heater is on high and the switch is in the defrost position, the circuit is pulling very close to the rated maximum of the fuse, and if there's a loose connection there, it will melt the fuse in a few seconds, as I found out after replacing it. Tightening the fuse holder fixed my problem... I suppose there are several other things that could cause this problem, for instance the climate control unit could have gone south, or the switch could be defective. If you have a voltmeter, get it out and check the voltage on both sides of the fuse before deciding it's not the problem!
Good luck with it.
'82 300D/4.3L V6