This is could be the wire that goes up to the ceiling lights, I would have expected to tap into the 10awg up higher but this they might have perceived this as easier. If you look at the manual you will see a couple of circuits that are tapped into the 30 amp driver side circuit. (Ceiling Lights, CB Radio) theses are both 18 awg. The ceiling light fuse is under the front panel where the wires go up to the penthouse roof. The transition fuse 10 Awg to 18 Awg in my mind should be closer to that transition.
Of course it could be one off customization, but it should still have a fuse for the tapped smaller gauge wire (unless the 10 awg circuit fuse is small enough to cover the 18awg wire. It is probably 30-amp, which is to high for the 18 awg wire)