If you're not familiar with
electrical work, this isn't an area to
DIY.
What I see is a "cigarette lighter" style outlet with only one wire. It needs two. Hence the word "circuit."
Since these can draw a lot of power, you want appropriately-sized
wiring, with the correct size fuse, from the source (battery.)
If I had to guess I'd say the black wire is ground. The heavy red wire probably was originally attached to the outlet, but was re-purposed to provide power to the gauges, probably because the original wire failed. But we don't know what else might have been changed, or why this was done. You'd have to trace all the wires back, and test to confirm any "guesses" like these.
If it were me, I'd pull that outlet. Not many things use them any more. They sell USB outlets that fit in that size hole. Everyone is always looking for one of those. After confirming that the black is ground, and the red is a good positive, run the wires to the USB outlet from those two. Include the fuse which came with the outlet, or add one, of the size recommended by the manufacturer.