The orange wire on a mercruiser is IIRC the alternator charge lead.
It would connect to one side of an ammeter and the other side would be bat+

In an outboard application I suspect that it would not be used.

I believe orange is generally 12+ for accessories. I know my old Grady has orange providing power to all of the fuse holders and there are a few orange wires that run to stuff like the wipers.

IMHO, if everything is working the way it's supposed to be working, I'd just ignore it. There are always extra wires running everywhere on a used boat. Impossible to know what they were used for without a manual from the maker. As to that large wire, could be for an anchor windless, or a trolling motor or anything else the requires a larger gauge. Might even have been for a spare battery. :head:

Wiring is all done with new block and all new switches. Orange wire is unused and the heavy gauge was for a trolling motor which i found out after talking to previous owner, actually supposed to pick up old trolling motor from him. other than running new wires for the float and bilge and some new connectors for the nav im good.

Just out of curiosity any of you every use a $5 illuminated switch from radio shack. i picked one up and am using it for the courtesy lights. See how long it lasts?

Here are the schematics.... not sure what year.