There are so many different ways of doing this. Whole seat swaps, putting heaters (junkyard or aftermarket) in your existing seats (or replacement seats), even aftermarket slip on heated seat covers. Each has there own wiring needs. Any of the aftermarket will have instructions on wiring. Factory heaters can go from a simple switch all the way up to computer controlled. If you go this route look for a vehicle with a separate seat heater switch and get that as well.
Personally I found something out of a junkyard that had a simple Hi-Off-Low switch. Pulled the elements out and put them into my existing seats. Looked up the factory wiring for the donor and copied it. I got another one once and found the switch went to a computer and that ran the heaters, gave up on that one, too much complexity.