I don't have a boat that I've done this to, my new one is still with the problem. You need whatever the length from your batt to your dash in 8 gau. red and black, an inline fuseholder (big car stereo type, 50 amp) and a ring terminal. Put the ring terminal on the red wire, connect it to the switched side of your Perko switch. run it about a foot or so to wherever you can mount the fuse. I mounted mine on the side of the bilge, up high. I prefer to run the rest of the hot lead (battery to dash) from the top side. Behind the driver kick panel is a hole that boat controls and wiring pass through to get to the engine, etc. attach the wire to a straightened coathanger and push it down the hole. WIth a flashlight, peer through the forward bow access under the bow floor, use another coathanger to grab the wire and pull it through. Run the black wire the same way, but no fuse, mount it to the battery black terminal.
Mount a marine dual terminal strip under the dash. Land the black and red 8 ga wires there. From two of the terminals, run a 10 ga wire to the two posts on your DECC box. If you do not have a decc box, you have the old style dash. I am not sure where to go from there but I'd look for wherever the black and red black 10 ga wires from the engine harness go, there must be a terminating point.
This harness comes out of the bilge, its about ten large wires, a couple of small ones. One is the grey tach wire. The red/black wire in that bundle come from the factory inline fuse near the Perko switch.
