Review on DOT Certified 4-Inch LED Trailer Tail Light Kit: Red and Amber Lights with Grommets, Plugs, and IP67 Waterproof Design - Ideal for RVs and Trucks by Aaron Schultz

Whatever other reviewers say, don't make the holes too big for these lights! measures 4" and 1/2" holes. After cutting the hole, insert ONLY the rubber grommet (the large black ring that surrounds the light) into the hole, making sure the grommet "seats" properly and the small flexible flange on the grommet covers the back of the hole cleans. This flange pushes through the hole, but it requires a bit of wiggling and tugging to get it all the way through and seated properly. Apply some pressure to ensure the light is fully inserted into the sleeve. To remove the light, push from the back (if you have access) or gently pry the light out of the sleeve with a plastic front cutter.