wiring harness under the seat
 

There's a wiring harness under both front seats of my focus which are plugged into something, what are they for, mines and LX spec if that helps and doesn't have electric seats or heated seats

Seat belt pretensioner. If your air bag fires then the seat belt buckle shoots down about 3 inches to tighten the belt hard and pin you into your seat!

Cheers, was baffling me lol

Do you have side impact air bags in the seats?
If you do the wiring will also be for these as well.

Not as hard as im aware, as far as I know just front airbags