My Grandfather did a nice job running and installing blinkers and tail lights but the wires are all very exposed (rear picture below) and I'd like to change out mounts.
If I want to swap out these wires with longer versions to run hidden (under side or behind seats) what gauge is best? I read 12-AWG is this correct? any recommendations on eyelets?
14 is what I used. Synders has vintage cloth covered wire and fabric loom in their Model A section. Looks better than the plastic stuff.
Lighting wires are #16. You can get proper-looking cloth coated wire and avoid the tacky plastic stuff.
There are some pretty nice, non-destructive, removable turn-signal/brake-light/tail-light options available from the usual catalog sources.
Once you've got the lamps mounted, you can choose to do the heretical thing and drill holes so as to hide the wires between the sheet-metal and upholstery (and if you do, use heavy-gauge wires that won't get hot), or you can avoid putting non-historical holes in your historical vehicle and instead, run the wires on the outside. -From a purist's point of view, turn-signals don't belong on a Model T anyway, so a neatly executed, short run of wires won't make matters any worse.
As you can see in the above photo, I used individual wires, but they'll would look much better once I encase them in a fabric loom (such as you have on your headlights). -Though historically incorrect, they'll be neat, unobtrusive, and to a non-expert, will look correct.
Side lamps on an electric-starter car like yours wouldn't be historically correct, but they would be period-correct and look better than modern, amber discs. -Empty, steel replica side-lamps are available and would be just the ticket for installing turn-signal bulbs. -You might want to pick up an extra for use as a brake-light housing. The more lights back there, the better.
Because my side-lamps didn't come with wick-knobs, I was able to avoid drilling the precious metal by running the wires through the existing wick-knob holes.
Even the very short, discreet run of wires between the lamps and the upholstered wall would probably send the purist into a fit of apoplexy, but that's his problem. -You can put safety first and still present a nice, neat appearance.