How to fasten these bolts?

slacken one of the screws by a few turns then try moving the screw in. If you can't move it in then the rear nuts are captive to the body. Remove screws and apply a small amount of thread lock.

Thats a good idea. If no one knows for sure I will give it a try.

There is a threaded plate on the inside of the body that both bolts thread into (like a sandwich - threaded plate, door jamb, strike plate - all held together by the bolts). Remove one bolt, apply threadlocker, reinstall the bolt ... then do the same for the second bolt. Don't remove both at the same time or you'll drop the threaded plate into the door jamb.

see what I am referring to.

tighten them, and they are loose again in a couple

and then do the other.

will lose them when I remove the bolt to put threadlock

else, how do I gain access to the backside?

Thanks