As I recall it, that gauge is switched on and off by a relay mounted next to the starter and GEN 2 relays under the dash. This relay should activate when the key is on.
Now, I don't understand why it was done this way, but that relay switches the *ground* connection to that gauge. So, as others have noted, there is *always* 24v hot on the back of the gauge.
If that gauge finds another path to a ground, such as if another gauge was installed next to it, it will be "on" all the time.
Of course, the needle could also just be stuck.