I got 3 pt seat belts from Andover Mustang. My question is how do I mount the 3rd point, theres no backplate for it. Should I just buy a back plate like the ones that go under the floorboards and use that? The 3rd point being the one that mounts at the shoulder height. I know many people use this setup just wondering how you did it...i've got all the other points figured out, just not that one since the kit didnt come with anything