aftermarket volt/alt gauge wiring help

The positive wire needs to be on a switched power wire. (a wire that is only 12v with the key on)

That's why I suggested to connect it where I did in my post.
I also prefer that it is connected to a good capable source (Large positive wire or terminal)
Other loads on a circuit using small wire like a cig lighter and other possible accessories could give you a lower than actual reading on the gauge)