Loose stabilizer bars

Noticed 1 or 2 of the stabilizer bars on my unit were loose, so decided to see if tightening the bolt would correct the problem. One right away tightened OK. The second one, the bolt just turns and turns. Decided that the nut ,,, on the other end of the bolt ... not the one with the wrench ... had worked loose and needs to be held by another wrench while tighting the bolt.

Only thing is, can't see how to get to the nut. There is a small plate the bolt goes thru that is mounted to the side and is held by 2 screws. Took those out, and accomplished nothing. Next, looked behind the plastic trim moulding. Again, nothing.

Looks like the nut end is inside the foam insulated wallboard.

Has anyone else had this problem; and if so, how did you correct it?