Mark, I hardly remember doing this install job.
I did it to my old trail truck back in '03 and I sorta remember that drilling the holes was pretty easy---getting the backing plate in and secure was another matter.
The printed instruction sheet probably had a template for where to drill the 2 holes.
IT looks staraight forward, but I don't have that backer plate. Everything else is new though. I'm going to try and find a way to mount in the D90, as I really don't keep my dogs in the back area, and it is a SWB so no room back there! Would be more practical in the Defender as I'm trying to keep them off those new PUMA seats!