There isn't one.
The block heater was intended to be a part of the equipment package, but it was later dropped. From my discussions with way to many people @ LRNA the best answer i received was that even in cold climates (Canada), the vehicle was able to surpass the emissions standards allowed without a preheated engine. All of the paperwork was printed and what not, and there you have it. From what I am told only a few early 2005 Canadian models have it.
We've fooled with using one from an S-Type, (same motor), but the project lost steam. Maybe as the cold approaches, I'll see what I can make work.