Heating pads arrived today. I measured resistance and came up with three power settings.
Stage 1:
U=12v
R=11.3 ohm
I=1.06A
P=17.7W
Wiring diagram for stage 1
Stage 2:
U=12v
R=7 ohm
I=1.7A
P=20W
Wiring diagram for stage 2. Resistor 4 ohm, minimum 25w, 40w recommended.
Stage 3:
U=12v
R=3 ohm
I=4A
P=48W
Wiring diagram for stage 3.
Sorry for my bad MS Paint skills