I looked into this when I autocrossed my old car. If the car will still be used on the highways (MVA definition), it requires all factory installed safety equipment, which means seatbelts. There is no mention of seats, so I left the rear seat out when heading to Pitt Meadows.
If you want to make it a truly dedicated auto-x car, there is tons of weight savings you can do if you don't care about which category you race in and won't be driving it on the highways. You'd be trailering that thing around though.
Originally Posted by Spidey
Why would you still need seat belts, if the seat is removed?
Because if the vehicle is used on the highways, you're required to have all factory-installed safety equipment (airbags, seatbelts, the bumper stuff underneath the outer facia, etc).