Not many. You could get them if you did the European delivery and had your car custom built with them but for the US market. Also, Boston Coach made a handful of V70s that were outfitted with them.
I have seen a handful of 98 S70 GLTs with rear heated seats, and one 98 V70XC with rear heated seats in my travels. Not sure how these cars were ordered, but perhaps they were early builds that came with the option included with the winter package. Initially we thought the Boston Coach models were the only ones with the option, but it has been proved false.
I've installed a kit on my car, and packaged two kits for sale on eBay in the past. They're not terribly difficult to install, and kind of fun in a way. As long as you're not bothered by a little upholstery work (lots of hog rings to deal with on the rear seat leather) then it's a nice weekend job. You just need to peel the skin back, apply the element with foam spray glue, then button it all back together.