Air Conditioner Fan

I have a 1996 Tioga and the air conditioning fan will only work on high speed. I checked the voltage under the intake filter and when the fan is on high I have 14 volts on gh terminal, but on low I have no voltage on gl terminal. Is it usually a problem with the thermostat? Is it ok to switch the terminals on the board high speed fan terminal and the low speed?

Thx.

Roger