So when I first got the car, first project was to add a hitch to it. While doing that, I noticed that the previous owner overtightened the lock nuts such that the mounts had shattered. There was enough material to still hold the lights in place, so I then decided to postpone the repair. After a year of driving, the mounts have been further compromised so I had to take care of it finally. I decided to test an approach using the right rear tail light.
So I used the following:
- one washer per mounting hole
- Fiberglas strand filler
- plastic utensils
I removed all weak material, mixed up some 'glas and laid a small layer on one side of the washer. I then sandwiched that wet layer between the remaining plastic surface and then added more 'glas all over the mount such that the backside of the mounting area was fully covered using the spoon. I kept it somewhat thick, but not enough where it was change the dimension of that area so that it wouldn't sit flat against the body. Any excess that squeezed through and onto the side of the washer where the nut would sit can be removed while still uncured can be removed with the tip of the knife.
I let it dry overnight and then re-installed. It's been two months and it remains solid, so I'll repeat this for the left light next month.