W124 front seat track bolts missing threaded body nut.

[Brian McL]

I am swapping front seats on a W124 and went smoothly without issues....pretty easy. But as luck would have it the last bolt to install was the rear outside one on the drives seat and the threaded nut in the body the bolt screws into is GONE !!! Obviously a captive nut...does anyone know how to access the underside of the floor that supports the seat track? It is not accessible from below...there isn't even a grommet to remove to access the nut. There must be another way...unless they installed it in the sub-floor then welded the whole thing to the outer shell. I don't believe Mercedes, back then in 92, did things that way.
Any help so I don't start ripping the interior apart unnecessarily.
Does anyone have the diagram of the assembly.