I have been looking into retractable seatbelts for my sedan ( coupe seats fitted)
I can get aftermarket belts but they fowl on the reclining mechanism and my worry was that belts would not be running vertically.
If you are fitting inertia belts to the rear of a coupe you may have to fit a bar between the inner rear pillars- I would check with your local seat suppliers as they would know.
hope this helps

tell us what you did in the front, a good retractable belt setup that wont lock up would be very nice

I'm in the process of fitting new belts in my 1200 Coupe and I have succeeded in the front but am having difficulties in the back. I had no belts in the back seat previously and have found mounting holes for lap belts but was wondering if I could install lap sash belts. I was wondering what the purpose of the two wholes with threads was on the walls of the coupe's boot space??? My bolts are too big for the threads.