I took a few hours today to get the small hairdryers installed.
In order to have a convenient switch to activate the hairdryers, I hijacked the defroster knob switch. I was lucky enough to have this option on my car. The wires from the defroster knob sit in their own plastic tube, so it was easy to find the yellow/red wire going towards the center wiring column of the car under the clutch pedal. This wire provides 12V when activated, just the thing I need to fire up the 12V DC relay.
Here's the travel hair-dryer attached to the heater intake on the passenger side of the car. The coupling is just short enough to let the hairdryer not interfere with the hood spring. The pipe clamps make the whole thing very snug.
Here's the DC relay mounted on the firewall just behind the middle battery box above the foot pedals. I used two 10-32 rivnuts to bolt this thing in. The two hair-dryers are tied into the terminals on the left. The negative battery cable is on the lower right (black wires) and the upper right comes from the positive battery post in the fuel compartment with an inline 10 amp blade fuse.
Remember the yellow/red wire from under the dash in the top picture? It's tied into the top coil terminal on the relay here. Since the chassis of the car is ground, I simply tied the bottom coil terminal to the mounting bolt (small blue wire).
For testing, I started with one hair dryer on the "low" setting with 5-amp fuse. It ran well (albeit loudly) and provided a slight warming into the cockpit. If I put the hairdryer on "high," the 5-amp fuse blows immediately. The hairdryer is very loud on "high" so I put both hair dryers on their "low" settings and used a 10-amp fuse. I suppose in desperate move to heat the cockpit, I could put in a 20-amp fuse and run one hair dryer on "high" since the high setting pulls about 15 amps. I'll try the "low" settings for now and see how it goes.
Next up: experimenting with an optocoupler to capture voltage for the "fuel gauge"