Just a little theorie from my side, I might be wrong I might be right?
As far as I know, a cooling system with a thermostat in it should work like this, isn't it?
little circuit includes heater system in the truck, That's the first one to warm up when engine runs.
Big circuit, is after the thermostat, includes the radiator.
engine starts, warms up the little circuit first, including heat in drivers cab. when that circuit is hot enough, bit by bit (by means of the opening and closing of the thermostat) it heats up the big circuit, which will then be cooled down by the outside air passing through the radiator, right?
I'm not sure if by our engines the sensor who provides the signal to the dash gauge is in the little circuit? but I don't see why we should need cardboard or winter screens before our radiators? if this cooling circuit (In total) was working ok, in real cold weather the little circuit would keeping the engine warm enough, while the big circuit would stay cool, wouldn't it?
Or am I looking over something?