Help <trailer guys>

[JLDickmon]

pull the lip of the gasket through the opening with a length of 550 cord wrapped around the gasket groove and lubed with you wife's dish soap.

kinda like you would do if you were installing a rear slider window.

 

[BlueMule]

Bushpilot, What has worked for me in the past is to put the bulb halfway into the gasket then start at one end (whichever way your lights go, mine are vertical) and press the whole thing together working down (or across) with the last little bit being a "thump" with my hand.

BINGO! We have a winner.

That's the way to do it. I have the oval ones on my trailer and all of our trucks at work have the round ones. Over the years I've probably changed several dozen of them and they almost always end up with a final "thump" to seat the bulb and grommet.