as far as the "overflow" bottle, it is a "coolant recovery system, where the fluid that exits the cooling system due to 'temperature expansion' (blow off from the radiator cap when hot/warm) goes into the plastic bottle, and when the engine cools down, is the same amount of coolant fluid is returned to the system when it cools down, both egress and exit from the cooling system are through the neck of the radiator and the past the pressure relief cap.
the local dealer should be able to provide you with the wiring diagram. it may not be all that much help, since there are likely to be 'magic boxes of smoke and mirrors' with names that may not instantly explain the boxes functions, however these trucks are pretty simple.