There are two drivers side fender-mounted cab heaters for the M37. The version that plumbs into the cooling system (like the pictures posted), and a gas-fired heater that sits inside a metal shroud and has the exhaust routed up the left side of the windshield frame. If I recall correctly, the coolant heater was supposed to be issued in climates with temps down to 0, the gas fired in temps below that. With the truck standing still, the gas fired unit on hi will cook you out of the cab at -40.
The passenger-side heaters are the powerplant heaters. Again, two models - a modified WC series heater kit that sits inside the fender, and a M37-specific unit that sits between the fender and running board and uses the keyhole knockout. THese heaters both heat the coolant (there is a modified floorboard to allow the coolant lines to come through the keyhole but stay below the floor) and have a loop that passes under the batteries to warm them, and the exhaust is ducted to a shroud around the oil pan to heat that. Also included were insulated hood blanket, side panel covers, and grille cover.
Another heater kit was available for heating the bed area when transporting troops. This unit was also gas-fired and mounted underneath the drivers seat. A 4" hole was cut through the cab and bed to duct the heat into the bed. Exhaust was through the floor and exited in front of the LR tire. THis kit also included a insulated cargo cover, insulated floor panels, and insulated back wall assembly with door to replace the tailgate.
