I have not done this on a C dory, but on may other boats. You have to remove bow railing. drill a hole in the center of the plate into the tubing. Be sure and polish up the edges, and maybe even put a gromet in the opening, or put heat shrink tubing around the wire where you go thru the SS plate on the bottom. The hole thru the deck, needs to be epoxy lined--route out the core of the deck (good time to fill the screw holes also).
Drill the hole in the tubing, under the place where you mounted the light polatform. The wire has to ge into the furthest aft tubing-so it will pass directly to where you have the platform (if you use one of the further foreward deck mount tubes, you will dead end where it is welded to the aft tubing.
Be sure that this is fused separately, maybe even after the switch.