Wow, that's a lot of great info, thanks. Can you tell me, is that info specific to my 78 300D W123? Most or all of it? I've quickly learned the big differences between chassis types and even individual years. I see you have a 77 300D, which I think is mighty, mighty similar.

I may sound slow on the pick up here, but I'm still struggling with one conceptual notion. I don't see how any of these vacuum or related non-electrical things would prevent the fan from blowing when I push in the buttons? I mean, isn't the button closing an electrical circuit and that energizes the fan?

I can see with everyone's help that where the air goes is controlled by a vacuum system, and even whether it is blowing cold or hot is somehow related to a vacuum system, but it just seems the fan would engage just by pushing in the button. I COMPLETELY understand this is just my ignorance of the details of this system.

Anyway, the defrost must work because it's combined electrical/vacuum circuits are not connected to temperature sensors or flap controls in any way. I guess when I push that button, the system know it is only going to blow hot air through the defrost and side vents. This lends credence to what I'm being told about the problem being in the system that determines temperature. Even with the button pushed, it can't act because it doesn't know whether I'm asking for hot or cold...maybe.

I really appreciate people helping to get me up to speed. It takes time, but when I get things, I've got it! I'm the kinda guy who likes to really know how the hell things work. I'm 100 miles ahead of where I was a few days ago!!!:)