The bumpers are held to the rubber mounts by L-brackets that sorta wrap around the top and bottom of the mounts. The mounts have raised bosses on the inside on top and bottom to keep them centered. If you had the mounts and bumper off the car, you would slide each mount in from center and then bring the unit to the frame to slide the mounts in and bolt on. The problem is, due to corrosion, the L-brackets (usually the bottoms) fall away allowing the bumper to easily come off the car. if you don't have matching 4 matching brackets on the inside of your bumper get a new one and take the old one to be recycled. I'm on my phone now, but I have a picture of a bad bumper on my computer, if you still want to see it let me know, I can post it tomorrow.