Depends on the car. FFR has cutouts in the rear wall for the straps. If a previous builder decided to omit them, you’d just slit the carpet and pass the straps through and then likely cover the area with a trim bezel
But be aware using shoulder belts w/ lap belt but w/o crotch or sub belt can be really bad. In formula type cars w/ a laydown driving position, a sub belt actually keeps the driver from submarining out from under the lap belt. In a car w/ a more vertical seating position the crotch belt keeps the lap belt from riding up into one's belly in an accident. Lap belts should be restraining our hips. If it rides up and restrains one's belly, there are a bunch of internal organs that can get badly damaged. My belting up process is get all the belts connected together. Tighten the lap belt like crazy, tighten the crotch belt like crazy, tighten the shoulder belts a little less. An experiment w/ not using the crotch belt is easy. Tighten your lap belt and then tighten the shoulder belts. See where your lap belt is grabbing your body now.
Craig is correct on this. 5-point belts should be mandatory.
Another part of the puzzle/problem comes from the seats. With the lap belts riding over the side bolsters the belt is fairly close to horizontal rather than the "U" that it should make around your hips. For many no matter how much the belts are tightened you are still not really secure. If you look at the seats like the Kirkey race or vintage lines, they have cutouts at the base of the seat for the lap belts to better wrap the belts around you.
Be wary of the 5 point on the street. Now you have locked yourself in and in a crash your head will snap forward. So wear the 5 point and your HANS device (like we do while racing!) and you will be safe.
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