I dont think anything involving heaters is a good idea, especially for things that will turn on and off (hot and cold) constantly like a carpc. It's better to just design something that will run in the cold and keep the system in the cold (eg. trunk).
My carpc is installed in the trunk where it never gets the chance to warm up to cabin temperature. I once made the mistake of leaving the back seat down in -40C while I drove, and after a while, big chunks of snow started to condense on my carpc and dangerously close to the air intake of the case due to the warm cabin air + passengers breathing.