Installing heated seats. How to power, wiring question?

As I don't have a Gen 3 I can't directly help, but Toyota's wiring diagrams are really complete. If you look it up on techinfo.toyota.com you'll be able to see the existing seat-heater fuse you mention and where the existing wire goes, what color it is, etc. (Unless they use a different wire harness in the non-heated cars where that wire is missing, but then you'll still see what terminal on the fusebox connector that is, which probably will mean you can buy a matching terminal, crimp it onto your wire, add it to the existing connector and use the stock fuse.)

While you're on techinfo, be sure to check in the service manual for any special procedures when you've had the seat or the seat upholstery removed. In later model cars the passenger seat occupancy sensor is a lot more than a switch saying there's weight in the seat or not. It actually tells the airbag computer whether the passenger is adult or child, male or female, and sitting back in the seat or leaning forward, using a combination of total weight, weight distribution, and distance between pressure points. I'm not sure whether they were that sophisticated yet in 2011, but if they were you should find a whole calibration procedure in the manual for after doing any work on the seat. That might also mean if you're using a generic heater membrane that isn't from the Toyota parts dept., it might be hard to get the occupancy sensor calibrated right.

Hope this helps,
-Chap