In the absence of a complete step by step write up for you (I'll try to get to this in the next week or two...), I offer a photo "journey" through the process. Including a visual warning to watch out for the sharp metal edges of the sheet metal in the seat! I got careless...
One thing I can say is that if you are quoted a large price from a custom upholstery shop (or similar) for this job, it's for good reason! It's not hard, but it took me most of the weekend.
I bought the heaters on ebay) for $138.74 shipped. (The seller's listing has ended, but they have the same product for a similar price still.)
All of the photos are here, but the ones that count are these two for the switches:


My backside is nice and toasty now!
Cheers, Bill