I don't know specifically about a 1st gen. 55; I'm a relative newby with only experience on my own 58 Chevy. However, the 58 came with three tpes of heaters/defrosters. The one that I have is mounted under the glove box and will either blow hot air at your legs out of a vent on the side of the heater, or through a vent between the dash and the windshield that runs the length of the windshield, depending on the position of a slider switch on the heater itself. Now, does this work well as a defroster/defogger? No, because it is basically recirculating the moist air already in the cab, only at a wamer temperature that sort of keeps it from condensing on the windows. In fairness, I think that you are supposed to open the vent on that side to let air in from the outside (the intake on the heater is pointed right at the right side vent).
