I had the rear seat added to my 2003 V70 (worked into the price) It is quite involved, but could be done by a DIY person. It involved changing out plastic interior panels, replacing most or all of the cargo deck, removing the headliner, and I’m sure there are some misc. bolt on structural parts too. The hardest part would probably be installing the locking actuator in the lift gate, which also gets a new interior panel, and interior door latch. Works great for the kids, I definitely recommend it.