I just went through this with my 1985 300D. First and foremost, check the fuse at the fuse block. Then, change the fuse. I know it sounds silly, but the fuse in mine looked good and had continuity. The blower refused to work intil I replaced the fuse. Don't ask me why. I thought that Mike Tangas was nuts when he told me to change the "good" fuse. But it worked!
The second thing to check would be the mono-valve, it so equipped. It is located on the firewall. If the seal is leaking, it will short out and may cause the climate control to malfunction.
My money is on the fuse.