After wrestling with those three nutserts in the back, I came up with a strategy. But let me backup. The nutserts in the back are a bit more difficult than the side because of how thin the sheetmetal is on the side and how thick/reinforced it is in the back.
Using the Astro tool to do the side, I got into a rythm of it. Thread on nutsert to tool, line up nutsert into hole, give the Astro tool some chest bumps and the nutsert will pop right in nice and tight. This didn't work on the back so well.
I was also paranoid of enlarging the holes too much so the nutserts wouldn't grab very well. I found a method that worked though. I did the same thing as before, but held the Astro tool nice and level with the nutsert up against the hole and tapped in the nutsert from the rear striking the Astro tool with a hammer where the mandrel came through the body. It was pretty easy and was just enough force to get it seated.
Now, for the tail light cut out. This is a lot easier with the right tools. I ended up using my jigsaw for most of the cut. Did great, nice smooth circle. But you can see through the hole that there's a support brace behind part of the cutout and the jigsaw blade was too long to keep from bumping against that. I thought about going to get an air saw from Harbor Freight, but decided to try using my Dremel. Thankfully it wasn't too long, or the sheet metal was too thick and I had safety glasses!