The hand warmers I ordered came in today.
I tried to find some of the larger ones that had covered junction point connections.
Performance testing them I found that with 12.5 volts applied they almost instantly jump to 90'F. They draw 1.6 amps each.
The set I picked up has a switch with high and low settings that included a 2 ohm 10 watt resistor for the low setting.
The instructions recommend putting the warmers in parallel and running all the power for the warmers through the resistor for the low setting.
One resistor to one hand warmer only drops the power to 1.2 amps.
This is with full power applied:
I am thinking wire them up with 2 ohm resistor through a time delay relay with hold in so when I push a momentary normally closed lighted switch power is applied to the warmers the little light in the switch also comes on and the relay starts counting down.
When the relay times out it cuts power to the warmers and the light also goes out indicating that the cycle is complete, if there is still ice on the mirrors, just press the switch again.