How can I protect a 12VDC power source from high-resistance shorts?
The solution is to protect each cable with a circuit breaker or fuse of appropriate rating to prevent the cable from melting in the event of a short circuit.

Ground fault protection is to safeguard humans against tiny currents flowing from mains to earth through a high resistance person. Such a current wouldn't trip the cable protection breakers, so the GFCI trips. It's only useful for leaks to earth, but this is a common way people are shocked. It provides no protection from live-to-neutral shorts.

Your installation should be safe against a short to ground at any point. If you have a large gauge cable with a large fuse, that's fine, but if you then take a smaller cable from that, the smaller cable needs a smaller fuse where it starts. This is usually done in a distribution board, for mains, and a fuse box, in a vehicle. Small fuses plug into the big 12 V distribution panel, and the thin wires go from there.