Re: Thermostat with Hurry-Hots
You have options:
1. Simplest - pull the switch out of the driver's panel and mount it in the back. If you don't have passengers riding with you, this would mean you would have to stop and walk back to control the switches - not so good.
2. My company SUV has an overhead console for the rear heat, and an "AUX" position, which "empowers" a second overhead console for the back seat, and lets the passengers adjust their own heat instead. You could get and wire in a second Hurry-Hot switch in the back, with the outputs in parallel with the driver's switch. Add a "Front/Rear" switch on the driver's panel applying battery power to the front switch or the rear one, but not both. This is probably better than a thermostat, in cases where the thermostat location causes a delay in responding to the heater cycles. You know, delays causing cycles of too hot/too cold/too hot. The RV furnace in our camper treats us like that. And if I understand correctly, the "Hurry-Hots" are original heaters using engine coolant, so you would not be using them while shut down and sleeping.
3. OK, you still want a thermostat. We have to worry about the current to run the fan motors. The home thermostats that tip a glass tube so metal probes dip into liquid mercury could handle a lot of current, but i suspect they would be bouncing on and off going down the road. This means you need a thermostat with contacts, plus a relay to switch the fan current. If you can't assemble a system from parts, go to an auto-parts store and get a fog-light relay kit. Cut the wire from the driver's switch to the fan motor, and following the kit's directions, wire the "battery" side to the wire from the front switch, and the "lamp" side to the fan motor. Take the two wires meant for the driver's switch that would control the relay and the lights, connect these to the thermostat contacts, and you are done.
If you have two- or three-speed fans controlled from the front panel, it might get harder. If the resistor to slow the fan is up front in the panel, and there is only one wire from the resistor going back to the Hurry-Hot motors, cut that one to install the relay as above. If there is one wire for each speed going back from the driver's switch to a resistor in the Hurry-Hots, you have another choice to make:
a. Install two (or three) relays, one controlling the wire for each speed, and wire all the relay control wires to the thermostat contacts.
b. If the motors do not ground to the case, but have a ground wire coming out and attaching to the bus chassis, cut the ground wire and install the relay here, controlling all three speeds. This will not work if special relays have some internal wiring connecting the battery hot to the control switch hot, but will work on ordinary relays with both hot and ground for the control coils coming out on external wires.
c. Pick one speed, perhaps medium, to be controlled by the relay and thermostat, and cut that one wire. If the driver selects medium, the thermostat controls the fan starting and stopping. If the driver selected another speed, it would bypass the relay and run the fan.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.