Does your harness run clips or does it have to be bolted in? If it's on clips then I'd drill a hole in the bottom of the exhaust tunnel (just infront of the rear seats) and use a backplate with nut on the inside of tunnel and a harness mount/eyelet inside the car.
Or you can mount it to the standard seat... as mentioned though this isn't recommended. Most people are guilty of doing it though, SEAT included.
Take seat out (make sure you unplug the airbag wiring from under the seat) the inner mount for the standard buckle can be unbolted and harness bolted in its place.
Get a decent seat though, the standard ones really are rubbish!