I referenced the schematic from KTA services and added some of my own circuitry to enable the heater relay if and only if both the fan and heat settings are selected. I also added an optocoupler to enable an LED when power from the 144 volt system is sent to the heating element.
I found a wire going to the fan to be at 0, 3, 6, and 11 volts for off, low, medium and high. I originally used a transistor to determine whether this wire were energized. However, what happens when I'm driving down the road and the incoming air vent causes the fan to spin? So, I now use a voltage divider and a comparator. The potentiometer allows me to use the high fan settings for heat and the low for cooling-down the heating element.
The heat switch is located inside the passenger compartment. This switch closes when the heat setting is selected. The current entering the passenger compartment is limited by a resistor.
I originally used a PNP to power the relay. However, I noticed that the +12 line has a lot of noise. I believe this was causing the PNP to partially activate during voltage fluctuations. I now use an NPN to ground the relay.
The LED indicator circuit used to have a resistor connecting the optocoupler to the 144 system. I noticed that I had random flashes of the LED. I believe this was due to the capacitors from the snubbers energizing the optocoupler during voltage fluctuation. So, this design uses a capacitor to filter out noise and a Zener to ensure a minimum voltage. The LED wire now is a ground instead of a power source. I believe this will also help the issue with random flashes.
I do have one more issue. I realized that many of the components I used are not rated for cold weather. If this design seems to work, I will probably make a PCB and use the components rated for cold weather. Right now, I just hope it overcomes the noise issue.