[davepl]

After 40 years the (bigger) connector on my AC truck was a little melty. Bothered me.

To be clear, I think only the AC trucks use the relay to trigger the fan. I doubt GM did that as a comfort/cooling-performance thing, so likely the larger fan required it.

If it was wiring, or a short in the resistor or motor, it should blow the fuse. Make sure it is actually plugged into the correct fused circuit. Maybe a PO got tired of it blowing fuses and plugged it into the IGN UNFUSED which has heavy enough gauge to melt the plastic. Wild guess.

If I had to make a more educated guess, I'd look at the blower motor resistor.

I could never find out what the ohm ratings are supposed to be, I did measure mine, wish I'd written them down, but what was for an AC truck anyway. Certainly not zero or infinity, but either of those would be obvious. So I tried to think of a way the resistance would be low enough to melt the plastic but not trip the fuse:

Mine was a little corroded. I wonder if you get a fine accumulation of gunk, debris, and oxidization that it might cause the resistance to drop. That would give you the "similar speed on all settings" feature and cause the resistor to get really hot when on any setting other than High. Remember these are right in the air intake path from the outside air. They've seen a lot over their lifetime.

So that's my guess (this last part).