Well technically yes the problem did start after I installed a new heater (never had one before). However, that is when I found out my father in law who owned the truck decided it was best to remove the Thermostat and hadn't had one in the truck for years.
When I was installing the heater I purchased a 180 deg thermostat, thinking I would swamp out the 160 deg one, but when I opened up the housing, there was no thermostat.
Thats when he mentioned he would always have problems with the thermostat being stuck closed. So I may have stumbled on a pre-existing issue that he never solved and left it to me to stumble across.
