I installed a Willians 4-point harness...
It took a lot of work to do a really good job with the rear eye bolt positions.
One is
inside the drivers side rear speaker grille, with a slot cut in it for clipping/unclipping. I used an angle grinder to make a clean set of cuts very low down at the front of the grille... virtually behind the top back seat padding. I need to push the top bit of the padded foam down to get in there.
The other eye bolt is dead centre at the very back of the parcel shelf.
Both were placed so as to go through two metal sheets. They both have big, chunky reinforcement underneath the sheet metal, then the nut at the bottom (the nut helps a lot to keep you in your seat in a crash!!

)
DO NOT just replace the rear lower seatbelt bolts with eye bolts... if you value your spine. The rear straps should go straight back, horizontally.
The front two points are simple eye bolt replacements for the front seatbelt bolts.
Hope this helps! I was gonna do a 'how-to' article, but never saw another FTO with a harness fitted at the track, so didn't think anyone else was obsessed enough to do it!
- Rich