1996 GMC Heater Blower Problem

I have a '96 GMC Pickup. The blower motor only runs when I reach highway speed in the truck. Anyone else had the same or similar problem?
I've replaced the blower switch and still have the same problem. The motor is okay because it runs once I get the vehicle up to about 55 mph. Is there some other device that controls the blower motor?
It's cold here in Ohio and I need to get it fixed.
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