2000 blazer. I've replaced the blower and the resistor recently. I'm having intermittent blower fan on settings 1, 2 and 3. 4 is fine.

 

Check fuses. The wifes sonoma only had 4 only when we bought it, check a bunch, found a fuse burnt under the hood I believe, and now they all work. That s on the manual climate control tho, not the auto one. Might be different, I know on my blazer with the auto climate, it was the control unit itself on the dash.

 

I'm updating here.

I had replaced the blower, ran a fused wire from the battery to the relay. Everything seemed fine for a long whole then I blew a fuse. Replaced that. A long while more noticed that my fused link was newly burned up on the blower side terminal but no blown fuse.