Whenever possible I change into my base layers after I get out of my truck. Sometimes it is not possible, but most of the time I don't wear any of my hunting clothes, including base layers in my vehicle. I just wear my base layers to the tree, then change into the rest of my stuff once I am high in the tree (if I am heated up) or just right there on the ground if my temperature is at a good level and I am not sweating.