Today I went to work more on the marker lights. The marker light started yesterday was completed using a pair of rivnuts. That worked well. It was all sealed with more of the magic seam sealant.
Following that it was time to look at the second and third marker lights. The second one which was in the middle looked OK but taking the lens off revealed a grounding screw that was so rusty that its head had dropped off. That left a stump to remove which came out after a few whacks with the pliers and some wiggling. After that it was just a case of enlarging the holes and inserting rivnuts then I had to put it all back together using my magic seam sealant.