Is there a way to access the underside eg via the shockabsorber mounting hole ? (would require removal of the absorber etc, so it would require determination, and/or great enthusiasm :-) )
/Lennart
Well, Lennart. It required a great deal of determination to get the old nuts out of the cavity, once I took the screws all the way out. Needless to say, the nut fell down and was out of reach on one side, but could be seen on the other.
After quite a bit of "fishing" and trying to pry the nut out, I finally had to get a file and enlarge the opening enough to get the nut out. It wasn't until I could get my Beltzer mechanics magnet tool in there, that I succeeded in extracting the nuts. See the various photos. As can be seen on the last photo, the nuts are indeed cage nuts, like the ones that Lennart describe. However, they are modified so that there only is half a "hook" on one side of the nut, as can be seen. On both mine, and yours too I suspect, the hook on one side has broken off, thus leaving nothing to prevent the nut from falling in. The nut itself is has been shaved in a 45 degree angle on the sides. I suspect that it is to allow the nut to be inserted from the top. With hooks only on half the edge and the nut shaved in an angle, I suspect that it may be possible to slide it in sideways and then twist it into place. As can be seen on the last photo, the nuts are different size, making me think that one side has been replaced. On that side the hole is also larger.
Well, I think that what I will do is to tack a piece of metal over the holes, drill a 10 mm hole, and put a 6 mm pop rivet nut into the hole. If it fails again, I can drill it out and pop in a new one. More after Jan comes back from vacation and I have access to his CO2 welder again.
Oh, by the way. There IS a hole into the space where the cage nuts are mounted. However, it is only about40 mm in diameter and located forward of the shock absorbers and accesible from the engine compartment. So it would require a very specialized and flexible tool to get a new nut in that way.