It looks like the aft upper bolt on my fender (the bolt above the upper door hinge) is either stripped or the nut is spinning. I can neither tighten the bolt or loosen it. If I were to guess the nut is spinning. Since the car is already painted (just recently) and the assembled I can't really see what's going on but if I were to guess the nut is spinning because of the feel on the ratchet. Is this nut a simple cage nut similar to the subframe mount nuts? If so, the only way I can figure to access it is to cut a hole behind the cagenut (inside and underneath the dash) and access it through the hole to that I can either hold the nut with a wrench or vise-grips or replace the cagenut with a regular nut. I really hate to do this because the hole will not be pretty or easy... but luckily it is hidden. Any other ideas or suggestions before I start cutting?

I have had to do this repair a couple times.
Yes you are right it is a caged theaded plate. You will need to cut an access whole from the the back side and weld up the broken cage.

Be sure to heat the bolt cherry red before trying to remove it. I see alot of people brake this cage trying to loosen rusted bolts.

Good luck !

Thanks Brian. I probably won't attempt to weld up the cage since the car is painted. I'll probably just cut enough access to hold the nut portion with vise-grips to tighten the bolt or replace the cage nut with a regular nut. The bolt should not be frozen up since it is a new bolt but I'm hoping the bodyshop didn't crossthread it.