[hazelhoff]

The rear cross channel is hollow indeed. Cinch nuts were used before welding the channel together. But that doesn't help does it ?
I had the same problem. Ended up taking three nuts and weld them on a peace of 1 mm. Sheet metal, whole pattern being same as the other side, matching the bracket. Make the sheet metal as wide as the height of the cross channel and long enough to stick out of the end of the cross channel. Now stick it in and do some fishing with one bolt. Then mount the bracket using another bolt (whole). Remove the first one and twist the bracket into place. Finsish the job off. Dealers were supposed to place secund rear lights in the early days ....