Is there any modification to keep the rear air/heat "ON"

The factory default is set to turn "OFF" the rear passenger
AC, Vent or Heat when the ignition turned off.

I don't like to turn the rear air on every time we get in the truck.

B

If the rear is not turned on when you use the rear fan control that is up front the rear uses the settings of the passenger. You do not need to turn the rear on each time unless the people in the back want to control it themselves.
Later
Leopardmech