I am not entirely certain, but I have heard and read many times that while the Valk std/tourer tail light
lens is the same as several other Hondas (1100 shadows, aero, ace, sabre), the fender mount bolt up is not exactly the same, and while they can be made to work it takes finagling, so I would get valk parts, not shadow parts. (I suspect if you take the time to look up part numbers, you will find the fender mount
part numbers for Valk and other Shadows ate NOT the same)
I also suck at auto electric, but seem to recall a piece of the std/tourer rear wiring harness is helpful, but not absolutely necessary to wire up the std tail light in the interstate fender knockout. I paid it done on one of my bikes, and had bought two of the small rear fender wiring harness pieces, but the installer only used one. (these are cheap)
The Radianz 128 led plug and play array for that tail light is a nice upgrade, I have one on my bike.
https://www.radiantz.com/VTX-Shadow-Valkyrie-Aero-Sabre-Ace-Retro-p/9012-11.htm (There may be a tiny bit of shaving required on the inside of the std honda lens, or not)
You can get converter boxes (Kuryakyn or Back-Off) to convert your rear signals to run-turn-brake (even on single filament 1156 bulbs), that requires finding red lenses for the signals (which exist, but can be hard to find). You cannot run constant-on amber lights in back.
Some guys have swapped out front Valk turn signals (dual filament 1157s) for the rear (single filament 1156s) (I guess they bolt right up), but that only gives you run-turn, not run-turn-brake.
Otherwise, the best way to get extra light in back of a supervalk is to get an LED array license plate frame. Since you're wiring in the new tail light, adding the lighted plate frame is not that much more work.
https://www.radiantz.com/Custom-Real-flex-Plate-frame-p/00184-lp.htmI would
not get an LED plate frame with with built in turn signals, two turn signals on each side is overkill.
