The third seat (rearward facing) was a dealer installed option, so it's a bolt in. The necessary holes should be there already. The kit included the seat itself as well as lap and shoulder belts.

I have a couple of them in red (one early style, one late) that I would let go real cheap, but I don't have the belts for them. You would have to find those somewhere else, as it would be unwise for anyone to sit back there without them.