By installing a relay, you are not running the wiring up to the switch, and then to the battery. I've completed a ballast system in an '98 MC PS205. I mounted 2 relays inside a 3.25" x 3.25" box next to the pump. The pump power wiring for the rear pumps is wired directly to the engine. Ground to the block, power where it goes to the starter. Here are some pics: