Remove the plastic thing around the hole, then mount up your light. Keep a screw driver in the truck just in case.
Something to think about.....some clearence lights will fit there.....some won't. My first one didn't fit too well and hit my tailgate leaving three 2" rubs of no paint on the bottom lip of my tailgate because the lamp was so 'deep'.
Here's what I've got on mine now.....TONS of clearence and looks ( .Y. )'s! It's a Peterson Manufacturing 169-3 in red. Peterson Manufacturing Company
Price is about $20 or so and I got it from my local sporting goods/farm supply store. Wal Mart carries Peterson lighting products too...idk about their LED line though.