Blower Motor,interesting problem

[72kool]

This morning i started my truck to warm up before work. i go outside after 15 minutes and it doesnt feel too warm in the cab. Not a big deal because it didnt frost over the windows. I felt the defrost vents and nothing.. i start driving to work and it starts blowing out the vents just fine. ok no problem i think. I get to work and check it when i park, no air through the vents again. Now the heat works well when it works. I have replaced the radiator and heater core and all hoses, all withing past 2 years. heat worked fine this spring without an issuse. it doesnt seem to have any coolant issue, just air flow issue. what would cause this? is it a voltage issue where the blower motor doesnt get enough to engage until im driving?