On my SJ I just tucked everything in behind the bumper location, but I also kept my lights on the bumper.
Instead of playing around with a relay, just go to a auto parts store and pick up a converter to plug in a trailer lights (Flat plug style, 3 wires + ground. Like for a utility trailer). Then just wire that into your new two light system. Something similar to
this should do fine.
If you go this route, you can buy two types. One will have your brake lights over-ride the turn signal. The other will do the opposite, if your turn signal is on it will not light up the brake light on that side of the vehicle.