850's have a built in accessory pass through for wiring. Pull the underdash panel on the drivers side and look for the conduit near the firewall just under the relay panel. Other end is located roughly below the brake booser under the hood. The underhood side has a rubber cap on the end with a few different size nipples. You cut the nipple for the size wire you are running and simply feed the wire through. There are also some accessory power conectors inside at the same general as the conduit. Look for red wire with yellow plastic conectors on the end. I believe 1 is direct battery feed and the other is switched (ignition) with a blank fuse in the fusebox. Happy hunting.