Thanks about the style.
You mean you were an electrical heavy line tech?
I'm not sure what you were testing to get "12v key off, .3v when running " and what your points were, but if you remember electrical theory, you would have a voltage on an open circuit, not closed. In a closed you get current, not voltage.
Also, in theory, a large wire decreases resistance and increases current capacity, and would be often desirable. That of course is in AC circuits. AC and DC have two different sets of physics.
In DC circuits, it could lead to too much current, as often a conductors resistance is calculated into circuit design to determine what size resistors are needed to reduce current to the capacity of what ever is being energized, I.e. transistors and Integrated circuits (chips) running digital odometer readouts, PRNDL displays, gauges, and the like.
A fuse is in the line to protect against a shortage, where something grounds out leading to excessive current flow and heat in the line which allows the fuse to melt, before something else does and catches fire. It would not protect against an excess of available current, which is always there, that a resistor then has to block. If they are blocking too much, then they overheat and fail (usually at the solder point first, but not always, depending on a few design factors) and that breaks the circuit open.
I don't know of any other form of step down transformer in a car that lowers voltage and limits current running between the engine bay from the battery and alternator, and the cabin area other than fusible links, and breakers. If anyone does, inform us please.
Another thing to consider, and this isn't really an electrical property issue, is using a house wire on a vehicle. Unless you used one with an outside rated sheath, house wires aren't typically designed to handle the weather that vehicle wires are.