Remove the rear footwell panels (assuming you don't have carpet), they just pop out; then at the forward edge of the plastics (where it meets the metal floor) you'll find two covers per plastic panel that pop up, remove the 10mm nut under each one.
Flip the rear seat bottom upwards and remove the T25s that hold the front and rear halves of the plastic covers together. Additionally, there is one T25 that holds the left and right forward plastic pieces together in the middle.
After this, remove the silver T50(or 47?)s that hold the rearward panels in place.
If you have a commercial (N1), you'll need to unbolt the rear seat bolts to gain access to the rearward T50s as they are under the upright portion of the seat.
The empty post on the busbar should be fine to power other devices (I use 2 empty posts for that purpose), however, make sure you place the appropriate fuse on the busbar, it uses a specific ZCASE fuse