A year ago I replaced a relay and got my fan to work on all four speeds:
https://blazerforum.com/forum/2nd-ge...rk-high-86003/

That lasted for a while and eventually my highest fan speed went out again. It did it in summer and I hadn't charged the AC so I didn't fool with fixing it and just rolled the windows down.

With cooler temperatures, I'm now faced with no fan at any speed. How do I troubleshoot this issue?

 

Disconnect the high blower relay, Jumper the purple wire to the red wire in the relay socket. Blower must come on high speed. Post your results.

 

Each time I've had a chance to check this, the fan's been working. I'll check back in when I have something to report, but thanks for the lead!