Preliminary Checks
WARNING: This page is about a different car, the 1996 Buick Roadmaster. However, it is still accessible from the selected car via links, so may be relevant.
Check HVAC MOD FUSE fuse (30-amp), located in instrument panel fuse block. Check for corrosion on connector terminals at blower motor and blower motor control module (A/C power module). See Figure. Repair components as necessary. Ensure ground connections are secure. If blower motor can be controlled in either manual or automatic mode but not both, temporarily substitute known good A/C-heater control panel. Retest system.