A 1957 suburban does not have back up lights. Just tail lights. I have had many of them. If you have the original tail lights, take off the lens and you will see the screws that hold the housing to the truck. Tinnerman and screw combo. If you have some sort of "back up" lights, post pix.
I think I see lights below your tail lights. Someone has added those from some other vehicle. Have you taken the lens of of those yet?
You need to learn to explore things like that by taking things apart.
Also, your fender emblems are on backwards.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn