I have been looking into this extensively, since I wanted seat belts both front and rear.
Belts with automatic reels can be mounted in the car. There are 3 possible places for the reel. 1) At the fixing point on the sill, together with the other end of the belt, while going through a loop attached to the fixing point behind the B pillar. Then this is nasty when getting in the rear of the car, some reels interfere with moving the seat forward and backward, and the belt will slide against the rear bench ruining the upholstery on the long run. 2) At the fixing point behind the b pillar; which is nasty since then the rear passengers can bump their heads while cornering. 3) somewhere else: some US cars have a fixture point at the ashtray position.
I ended up placing fixed seatbelts in the front, since I did not find a satisfactory place for the reel of automatic belts.
later cars have fixture points for the seatbelts. I posted some pictures of these points some time ago on this forum, so pls try the search function. Automatic belts can be placed using these fixture points, with the reels on the parcel shelve. However, since I installed fixed ones in the front, I also went for fixed ones in the rear.