I'm north of you and I do get this , have you checked all your weather strip for condition also check to see if the rubber weather seal is there for the inner door handle , one other thing you can do in remove the door panel and glue vapor barrier plastic to cover the openings in the door , GM does not do this from the factory, makes a big difference to stop some of the drafts coming out of the door , also try to do something with the gap in the door weather strip down by the floor, and check the fit of the door to the door seal , if you are still cold try a hot water heater kit also the arctic grille cover but at this point you should install a temp gauge so you know what the engine is doing, that should get you there,