As a further clue, does the Mini have rear seat belts (3-point or 2-point lap) that look like the standard factory fitted?
I don't think a '78 Mini would have the rear belt mounts? (Unless owner fitted later) Can someone confirm??
If your Mini shell is post-1980 it will have the lower 2 Point Rear Seat Belt mounts visible low down on the rear seat bulkhead, you will be able to see the factory welded threaded plates for them from inside the boot. From the mid-1980's there is also a top mount each side of the rear window but this is hidden by the headlining when 3-point belts are not fitted.
See photo here for mounts:-
http://www.theminifo...ounting-points/