Artemus,
Here is a little better picture. You can see the L and reverse L-shaped grooves at the bottom corners of the cap. Inside the grooves are two strings of blue led lights that are controlled by a switch inside.
Here is a pic of the lights on during the day, doesn't help much. but you can get an idea.
Bingo
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Bingo and Cathy - Our adventures begin in the hills of WV. We are
blessed by our 2014 3850FL Big Sky (previous 2011 3750FL and 2007 3400RL) that we pull with a 2007 Chevy Silverado Classic DRW CC dually.