If OEM is anything like my retrofit, then it'll be one fuse for both seats. If one seat is working and one isn't, it's unlikely to be the fuse but I would always check everything when troubleshooting.
As for cabling, the heating plug under the seat is the large black rectangular one. The medium sized yellow one is the airbag and the smallest black one is the occupancy sensor I believe. It's perfectly safe to unplug any of those as long as you don't turn on the ignition whist they are disconnected.
I've read several threads on here about heated seat elements burning out, which is a possibility. It could be that or potentially the wiring. You could check everything with a multimeter but probably worth getting VCDS on it if you have it or someone local that can take a look.
Best of luck with the troubleshooting mate. Electrics can be a real ******.
