There are many posts out there on this subject, so I'll try to stick to just additional info that would've helped me.
I wanted heated seats, but DAY-UM they're expensive! I decided to see if I could install my own. I bought a 2-seat kit on eBay with pads for seat and seatback. The instructions included were enough for the overall install, and I found a couple great tuts online, but here are some pointers:
-I used a 20A pigtail fuse tap to power both seats
-I didn't use the stock Heated Seat fuse location; I used the fuse location farthest to the right from it. It's empty from the factory, and is a switched circuit. (As I understand it, you need a relay to use the stock fuse location).
-I grounded the heaters to the bolt under the steering column that the OBDII is mounted on.
-Definitely remove the seats from the car to do this!
-When the seats are out, you can fish the wires under the carpet between the seat area and the console. There are already carpet cutouts for wiring. Looks OEM!
-You can drill 3/4" holes to mount the switches in the rectangular hole cover blanks in the console where the OEM heated seat switches would go, but be careful. They just BARELY fit.
Here's the cooler way to do it: use OEM switches! There are many models of Subies and Nissans that have these same switches. I got a pair on eBay, with a few inches of wiring and the wiring connectors, for under $40. Get ones with wire if at all possible.
I found some info online on this (thanks Fox!), but it was somewhat inconclusive. After a lot of testing and messing around, here's how it works:
First, you have to have the 4 contacts going to the right connections. The aftermarket kit has these colors of wires:
Red = Hot 12v
Black = Ground
Yellow = High heat
White = Low heat
The OEM switch, as you look inside it from behind, is arranged like this:
High Hot (12v)
Warning: test the switches! It appears that there are different switch models out there. This is how mine work though.
The OEM switch does not work the same way the aftermarket ones do. The OEM switch sends DC to the Low wire when set to Low, and to both Low and High when set to High. However, the aftermarket switch sends only High when set to High. This can be defeated in the OEM switch.
If you pry the outer shell off the switch, you'll find a traveling plastic piece with a copper "rake" on the bottom that slides on a circuitboard. The traveling piece also has a tiny copper bar along the top that comes into contact with 2 rails on the circuitboard when the switch is set to High. This is the piece that joins the high and low circuits together. Carefully cover this bar with electrician's tape and the switch will operate as the aftermarket switch does, including the indicator light (one color tho, not dual like aftermarket).
This entire mod cost me around $170- a far cry from the $450 I was quoted at a car customizing shop for installed aftermarket heaters, or the $650 from dealership- PARTS ONLY!
Please let me know if you have questions.