The cable from the seat belt pre-tensioner travels under the seat and joins the other seat cables (airbag, seat heaters, occupancy sensor) at a multi-plug that is fixed under the seat base - each individual cable has a socket on the end of it and they all slide into a small box so that one plug from the car's floor connects to all of them.
The photo below shows the empty box under a Recaro seat, but the parts are the same under a normal seat. You can't really see them with the seat in the car.
To take the seat out:
- disconnect battery and wait ten minutes, so you don;t risk setting off the seat side airbag;
- slide seat forwards and backwards to get at four Torx bolts holding the fixed seat runners to the car's floor;
- tip up the front of the seat a bit and the electrical connection is visible;
- to remove the connection, slide the locking part to the side and the plug will be ejected (the last two photos show the black locking part unlocked and then locked).