4" round lights on rear of sleeper

I'm trying to swap the 4" round lights on the rear of my sleeper and not having much luck. What is the magic trick? I have brand new grommets (Grand General) and I can't get them fully in. Do the grommets go in first or do the grommets and lights go in together?

 

Figured it out. Insert grommet ALL THE WAY then insert light.

 

The old lights were too hot and burning the insulation in the wall and the old grommets were petrified solid. Switched to led's to keep the temps down and got rid of the last owner's hackjob wiring. Also wired them as secondary tail lights.

 

Forgot to add that my truck is an old dinosaur (1994) and uses the sealed trailer lights in the rear of the sleeper.