Scania side lights help

[Louisbaby]

Hey guys, I want to install lights in the side panels of my Scania. I've seen a few Youtube clips with Scanias running side lights, but my question is this: the panel has little square engravings for the lights, do I cut out square and replace it with clear plastic and then back it with a LED? Or do I drill a 3mm hole and fit the LED straight to the side panel?

Also, do you guys use 3mm or 5mm for your running lights? I'm a total noob at RC Trucking, so please forgive me if this is a stupid question...

Thanks