The Power Heater comes with the Cold Area Package and the 1HD-FTE engine (4.2 L 6IL TDI). The vehicle model code ends with a W (Winter?).
Don't know exactly how it works, but the coolant runs through it, and it heats the engine up to working temp faster, while putting a fair amount of stress on the VV-belt.
I've noticed that now, during the winter season, normally with temperatures between -10 C and 0 C (freezing), I get a good heating effect after about 1 mile driving with the power heater on. Without the power heater I get the same heating effect after about 1.5 mile. So, the engine gets up to a good working temp faster, and the kids and myself feel better.
I know from living in even colder areas that a diesel engine produces so little heat when idling that it will cool down to well below optimum operating temp in the winter. That's why diesels have either a manual idle speed adjuster on the dashbord, or an idle-up button.
The Repair Manual is not very informative when it comes to the Viscous Heater. It tells you how to remove the unit, and how to take off the magnetic clutch.