The coolant comes from the head, and goes to the top of the rad, and then down to the Thermo. FWIW, blocking the rad doesn't really help the warm up much, it helps keep it up after you get it warm. Diesel's do not produce much heat, due to being almost double as efficient as their gasser brethrens. So having the cold air enter the engine bay and flow over the engine actually does steal away quite a bit of heat from it. When the t-stat is closed the coolant goes down from the head to the water pump and back up to through the engine. Coolant will flow through the rad when it is starting to open and fully open.
Be careful about covering the rad up, it'll be fine until you work it real hard then it'll overheat.