Run the wires around the outside of the bumper (I ran mine around the driver's side of the vehicle). One you get the wires underneath the truck, Use the double sided tape on the 4 clips that came with the lightbar. This is where it got tricky and maybe tough to explain through a forum without photos.
Use some kind of straight-edge (yard stick, etc.) and make sure you mount the 4 clips level. Once you have done that the lightbar should snap right into place. It will be a tight squeeze for your fingers but can be done without removing the tailgate.
One other piece of advice, my harness did not come with a connector for the reverse wire, so I had to run to the auto parts store and buy a coupler to connect the 2 reverse (white) wires.