Holes for taillight trim

[BrianG]

I want to add the stainless trim to my fleetside bed tail/reverse lights. I have a copy of the "Factory Assembly Instruction Manual". In there it gives the correct size of holes and position from 'reference' lines. I have no idea how I'm supposed to locate the reference lines, so I'm assuming I need to find a hole first and go from there. I'm sure that some of you must have added these to a truck that didn't originally have them. Any and all tips would be greatly appreciated. Thanks, Brian.