I'm sure there's a how-to for doing that, somewhere on here?
Other than that I'd say you're best taking a feed from the front 12V sockets and running it under the carpets to the back of the car. Or you may be able to get an adaptor that piggybacks a feed off the 12V socket fuse in the passenger fusebox?
Any heated seats wiring will only have power when the car is running and heated seats switched on, and wiring into the courtesy lights could mean you're on a lower rated fuse than suitable for what you want to power up with the 12V socket.
From memory the courtesy light wiring is pretty thin gauge, considering how easy it is for some 12V adaptors to short themselves out across the power socket and blow the fuse that's another reason for avoiding using a lighting circuit, imho.
Is it definitely a spare power socket feed that's already spare in the boot?
It's just if it was then I'd expect the 2 cables to be there 12V and earth in the same connector so that if it's utilised on other cars the earth circuit is the right spec already and you just plug the socket in to the connector.
Well I was right about 1 thing and wrong about the other
Wire in an earth cable and you're sorted :)