This weekend I was finally able to improve my heater, both in quantity of heat and reliability.
First off, I pulled my blow dryer heater coils out and found everything intact. Nothing burnt. Nothing funny. These coils draw about 7A at 75V, which is 525W. So what I did was remove two loops of coil to decrease the resistance.
Here is before:
Here is after:
For reliability, I now switch the coils on and off with my ex-field interrupt relay which is basically a Ford starter relay. This is instead of a regular old automotive 12V, 5-pin relay. Breaking high voltage DC current is quite difficult, and my old 5-pin relay basically had internal welding each and every time I turned the heater off because the contacts only open about 1mm apart. It is actually quite amazing that it lasted so long.
The results are good indeed. Now the heating coils draw 9A at 75V, which is 675W. A 28% improvement. Air coming out of the vents/defrost definitely feels warmer than it did before.