My car has an unfortunate lack of always-on 12v power points -- something I'd like to remedy. I want to add a new powerpoint in my center console hooked directly to an always on source, eg: the battery (primarily for charging my phone while camping). I know I should run wire directly to the battery and fuse it within 18 inches of said battery, but that's a pain. I already have a (properly fused) 4 gauge wire running from the battery to underneath the center console for two under-seat subwoofers. The wire is oversized for said subs (young foolish me thought I might replace them), so I don't see any worry adding additional load to said wire.
Would it present a fire hazard if I were to instead of running a dedicated hot line from the battery just stick the wire into the incoming port of the distribution block alongside the incoming hot wire? The distribution block is actually sized to accept anywhere between 0 and 4 gauge, so I suspect I'd have room for an additional wire for the outlet. My only (poorly understood, hence the question) concern is that the ~10 gauge wire for the new 12v outlet is much smaller, so it might burn up before the fuse on the main trunk. If that is the case, could I just get away with secondary fuse just on that wire?