I used 3/16 " aircraft cable and 2 quick links to connect my harness to the top rear seat latch in my ZX3. I run the belt through the head rest posts. It's
a good safe angle for the belt and it removes easily. It doesn't interfere with using the stock belts.
I picked up a free seat belt extender from my friendly Ford Dealer. Cut off the male end and insert it in the latch to stop the warning chime.
Nothing is fool proof. Fools are very ingenious people.