replaced fuse now only blows on high
ac blower blows only on high on 1999 Chevrolet 1500 Pickup
Blower Motor resistor needs to be replaced.
I looked to see if there were any recalls or technical service bulletins related to the blower motor. perhaps the blower motor is drawing to much current on high speed due to a motor that is seizing up or has wear causing it to experience problems electrically. I looked depending on whether your truck has air conditioning or not takes between it takes .9 to 1.4 hours to remove the blower motor to test or replace it. You may find leaves and debris in the system or ducts that may result in the blower having to work harder.
demologik is right, Thats all its going to be Ive seen it a thousand times