if you look at the crack is it just below where the vent is blowing up the warm - hot air might not be frosty but fast expansion on a cold screen i'v seen stone chips crack across the screen with the heater left on
we have large office toughened glass panels shatter here due to expansion
another item mentioned in here the heater box could get really hot too might be why some of these plastic control arm linkages break as people tend to turn their heater to full getting in a cold van. you had to in the old days... my knob lol is set to 22 ( mid way ) and heating comes on hot i never need to adjust it
im a ex air con engineer..... don' t touch the thermostats.... is why they are thermostats lol let them do the work.
