30 AMP HVAC fuse keeps blowing

I have a 2000 Chevrolet s-10 xtreme with a 2.2 manual. The problem is that when I place the blower motor control switch to level 4 the blower motor runs on high for 15 to 20 seconds and then blows the 30 amp hvac fuse in the under hood fuse box. I have already replaced the fan relay, resistor, and blower motor. All other speeds work fine.
I researched the schematic and what I found was that the switch bypasses the resistor pack in high mode and uses a relay to run the fan instead. I tried the shake test on the wiring and this had no effect.
I'm thinking there is a short to ground somewhere in the wiring between the control switch and the relay or the control switch itself is bad. Has anyone ever had such a problem with the control switch? Or is this a known common problem for this year and model?

I figured it out, I had origionally replaced the fan because the bearings went out. The new fan had must have had bearings going bad because as soon as I changed it the fuse stopped blowing.

Thanks for posting back what you found.
Glad all is well now.
MT