1st thing to check with the heater controls is has the D stem collar of the knobs broken? If so then you can turn the knob without actually adjusting anything, just pull the knob off……..NO not that knob! Pulling that knob, however pleasurable won’t fix the damned heater, though it may well warm you……..No the knob l refer to is the one on the heater control, pull it off the control stem and have a look at the stem collar on the back of the knob.
If the stem collar of the knob is only split then you can effect a temporary repair with a few layers of heat shrink tubing, as many as you can get on. Once you have new replacement knobs you can reinforce them with heat shrink tubing in the same way, they should never fail again.