Heater Fan and Switch Question

[leon bee]

Happy Thanksgiving! I'm working on a 70s land yacht that is gonna be my tow rig. By the time I got all the rodent debris out of the heater/ac/defrost ducting, blower, core, all that is now in the junk pile. Most of the stock controls were defunct when the car was parked years ago. I thought, hell, I don't need a heater......until I drove it on a real cold day. So I'm gonna buy one of those old style hang under the dash units and run a duct right over to my feet and a defrost to windshield. Realized I don't know how fan speed is controlled, but now I'm thinking I need a switch with resistors or something to step power down for slower speed than wide open. I don't really have anything here to look at and google confused me. Thanks!! for any info!