Ive been wanting to add some LEDs under my trailer so I can see my tires at night, and I personally think it looks pretty cool shining down. Anyways.
Where do I run my power to? Where would you?
Do I need a switch or anything that triggers it to turn on?
Is this legal? The lights are not super bright but is it legal?

Any help would be great

I know its not legal to run down the road with the lights under vehicles/motorcycles. Dont know for sure about trailers though. I'd wire them into the side marker lights, so when you turn your lights on, they would come on also.

Okay thank you!

that is the way I would do it. Make sure you do not over load the circuit. Add up the amp draw on what you have now. LEDs all around would be the way to go for lower draw. Not sure how well you are going to see the right tire. Also using lights that are bright enough to do any good might draw attention from LEOs. I know you cannot have white lights shining towards the rear but not sure about down. I saw a guy on 635 that had blue and I mean blue and not just tinted fog lights. I am certain those are illegal. Sometimes I think the LEOs just let some things go. certain those are illegal. I remember when you could not get red or blue bugs shields in a lot of states.

use amber and shine them to the inside and down under the boat and you are legal, same thing as marker lights. be sure you do not interfere with brake or outside clearance lights or you might get harassed.

Thanks!