The lever on the auxiliary heater indeed cntrols the amount of heat. I went
to a lawn mower repair shop and attached a short length of lawn mower cable
to this lever in my '90 Westy. Now, my rear passengers  can control their own
heat!

The white knob is a bleeder valve. Someone just didn't close it enough.

I relocated my rear heater over to the side so I could get a big space under
my rear seat where I keep my winch for getting unstuck when I drive
irresponsibly.r