Problem with voltage gauge

Sounds like a textbook case of a bad ground.

First, recheck your wiring. Any 12V source will work to power the gauge, so any fuse that powers something under the dash is fine. If you had to a source that wasn't supplying power, the gauge wouldn't move at all when you turn the key on.. so it's getting power.

Use a fuse that has power only when the ignition is on, like the radio fuse.. that way the gauge lights and gauge will shut off when the car shuts off. When they say "Ignition sourced 12v" they don't mean the actual ignition fuse, they mean one that switches on and off when the ignition switches on or off.

Second, if the needle still pegs when you turn the car on, I'm 99% sure you have a bad ground. This is years of experience troubleshooting talking, and pegged gauges almost always are bad grounds. Think about it. If it were the power wire, would the needle move at ALL?

Where are you using for a ground? What type of connector? Did you recheck the ground yet?