Good news everyone! If ya get a good enough switch, you don't have to run a relay.
120W on 12V draws 10A, something that is a stretch for 18ga wire but within tolerance of 16ga and certainly safe on 14ga, still a decently small wire. Cheap 15A switches, in my personal experience, will kerplode about a year in on a 10A circuit. I would get a 20A or fancy 15A switch if I wanted to avoid a relay, but you can go that route.
That said, personally I'd hook a 14ga wire to battery (or pdc) and run it fused to the 30 post, then from 87 post to the lights. Ground the lights (still 14ga). Then run a 20-22ga wire tapped from an available circuit (remember, we are talking well <1A on this wire) or battery/pdc (fused) and run it to the switch, then on to post 85. Ground the relay, post 86. Easy as Sunday mornin. Some parts stores (and amazon) sell relay harnesses that make it splice the wire, plug & play, as opposed to spade terminals on each wire.