Is the rear seat on a 1999 XJR the same as a 1996? I've been trying to locate a replacement bottom cushion in black, but I can only find whole seats(bottom and back) for a 1999. And I just need a bottom.

I believe they are different. IIRC, in 1995 and 1996 the middle back seat had just a lap belt, while in 1997 it was replaced with a 3 point seat belt.

I agree with the above post. The seat back and seatbelt anchor points are completely different and it would be a fuel tank out job to make a later seat back fit an earlier car. However, I think the seat base might fit. It does look different in so much as the earlier car had deeper recesses for the posteriors of the two passengers. The later car has an altogether flatter seat base that would be more comfortable for the middle passenger. I'm pretty certain the seat belt clasps are the same on each car. (And these fit through holes in the seat base) There may also be a small area of cloth rather than vinyl at the extreme left and right of the later car's seat base.