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.
Mike
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.