Anything that would have heated seats would not be a direct bolt-in replacement in a 1988 4Runner. Heated seats were never an option in trucks and 4Runners of the 1980's and early to mid 1990's. While mid to late 1990's Camry's, Land Cruiser's, and Lexus may have had heated seats, the seats won't just bolt in and there aren't any heated seat harness plugs on your 4Runner harness to tap in to.

I've seen generic heated seat kits on Ebay, and I've heard of people installing them on the ih8tmud.com forum (land cruisers). You might want to check those out instead. You have to remove the seat cloth/leather, install the kit and trim to size, and run the wiring. Otherwise, you can pick up one of those automotive heat/message cushions at Walmart for around $20-$30.