I have this other circuit design that looks like it could be modified
and adapted to actually giving real temperature control to the USE heaters.
Since the heater element runs on pack voltage this would be a high voltage
control with a pot on the dash that changes the temperature. The pot
would be opto isolated for safety. It would be just a single board
that connects after the heater relay to pack voltage, and uses high
voltage fets for controlling the power to the heater element.
Depending on the space available, it could mount in the heater box
itself.