I have had a problem with my HVAC in that it would only blow in high speed. All the other, including dual temp controls, worked. I changed resistor, relay and finally, the motor (because running it on high began to wear out the bearings). Nothing fixed it. Then last week I had the trans re-sealed with a new axle installed because of a vibration. When I got the van back, the heater worked! ON ALL SPEEDS!
I have not had a chance to talk with the guys at the trans place, to see if they found an open connection somewhere but it seems VERY strange that everything I DID availed nothing but they fixed it by accident!
John