In the process of running down the reason the heater blower would only run in the high setting. This information I discovered,,,,,
#1 The resistor is located on the outside firewall (gen slide open), high and on the passenger side of the heater core assembly.
#2 The heater controls are from 1997-2000 GMC Savanna (AC Delco #172224)
#3 The heater control circuit board had been "WRV'd" as E Mike would describe it. (3.8" sheet metal screw into board during assembly)
#4 The dash mounting area for the radio and heater control is very poorly designed. This allows the radio and controls to move enough to crack dash severely. The whole thing sags onto the hydraulic control rocker switches turning them on when the tray is slide in and out.