If you're going to be adding a few things, add a little fusebox, driven from a single +ve from the battery - it helps keep things tidy, and tidy is safe. Put any inline fuse as near to the battery +ve as possible, definitely before any potential chafing or sharp edges. If you take the wire through a hole, or past a sharp edge, or past anything moving or hot, protect it.
I'd go with a body earth, too - after all, it's how everything electrical on every metal-bodied car of any type is earthed...
As for wire colours, I'm not too anal (I'm used to old Citroens, and they seemed to just use one colour of wire with different colour sleeves at the terminals - all of which faded to off-white), but I'd love to find the person who used green-and-yellow for live wiring on a friend's camper van... <facepalm>