69 Dart seat belts?
Can't speak for all, but on mine and a few others I've seen, the shoulder belt attaches to the side rail of the roof above, the fixed point above the quarter glass and the buckle to a clip attached under the visor mount. You may not have to take down the headliner to find the rear attachment point, if you feel along that top side rail above the quarter window you should be able to feel the threaded hole where it attaches.
Can't speak for all, but on mine and a few others I've seen, the shoulder belt attaches to the side rail of the roof above, the fixed point above the quarter glass and the buckle to a clip attached under the visor mount. You may not have to take down the headliner to find the rear attachment point, if you feel along that top side rail above the quarter window you should be able to feel the threaded hole where it attaches.