Mine have never been used pending a roll cage fitment to give a mount bar. Currently they are fitted using eye bolts to the original mounts with the shoulder straps picking up on the rear seatbelt mounts in the back seat area. The lap belt needs a threaded reinforcing plate attached to the side of the tunnel as the OE seatbelt is attached to the seat subframe on the inner side, similarly the crotch strap needs either a reinforced thread attached to the floor pan (as I will do; they aren't in at all yet) or a bar between the 2 front seat base mounts with a thread for the bolt in the middle.
The reason mine have never been used is that the angle the shoulder straps take to get to the eye bolts in the back seat is too steep and would cause spinal compression in a crash. I will go for a full cage with a harness bar and mount the shoulder straps to that, but the alternative is to leave them where they are but fit some sort of guide bar using the upper seatbely mounts on the B-pillar so the angle from the shoulders to the bar is close to horizontal, then the angle down to the anchor points doesn't matter, so long as the bar is strong enough not to bend or break in a crash.
Last thing to consider is that with the original Porsche seat the shoulder straps tend to fall off the shoulders. I don't know if there is a satisfactory solution to the problem other than replacing the seat with a bucket that has harness slots in it.
I won't be tracking my Mazda until it has a roll bar with harness bar installed. It had seats very similar to the Porsche ones, but they are in the garage now and the car is fitted with some carbon fibre Recaros that have the necessary slots.
edited to add: My harnesses (Sabelt 3" 4 points with quick release buckle and 2" crotch strap add-ons) came with clips on the ends of the straps to attach to the eye-bolts. They could be quite easily removed in a couple of minutes if necessary, though I just hange them over the seat out of the way. The only downside of that is that the buckle hits the seat shell if the car is flung around, making a banging noise.
Lastly 1992 is waaaaay old for harnesses. They have a life on them for a reason and if they are second hand who's to say they haven't been used for their true function (which makes them scrap)? Personally harnesses are something I'd only buy new.
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