Well if it is an on/off switch it would not matter, The motor is grounded by it's sheet metal attachment or dedicated wire and the hot is broken by the switch. If it's a variable switch be sure motor is compatible. I don't see ANY heater on a dozen wiring diagrams for 47 I looked at. Obviously an accessory. Did find vague instructions to install heater as an accessory in 1951 car and truck accessory manual. Did not mention variable rheostat type switch, just on/off. Don't know history of your "heater" and "old switch" to tell if you have GM dealer accessory or other? Anyway the answer is that you will have the fused hot wire going into and out of switch, as you said. Either terminal. All a switch does is make/break contact between terminals. To calculate fuse: get watts or amps from motor markings or the internet and then calculate load/fuse/wire size. You can, of course, measure the current of the motor when it's running. BTW: 6V stuff needs larger wires than 12V.
