Volt gauge reading incorrectly, need some help.

[Custom/Ten]

Driving home yesterday and looked down at the cluster and saw it said 9V. I'm thinking my alternator must of went out and I hurry home and start testing the alt which tested fine. Tested the battery and it tested good too. My charging system is all within specs and everything is working good, starts good, lights and radio are good, no voltage drops. Only thing wrong is the reading on the volt gauge in the dash. Spends a lot of time at 14 like normal but also goes as high as 17 and as low as 9. It's the only thing in the truck getting these readings. It is reading fine most of the time unless I start adding loads then it starts dropping or bouncing but like I said it's just showing fluctuation in the gauge not on my multimeter. Unfortunately my computer is not working so I can't download any wiring diagrams in PDF because my phone can't do that. I need to know what the power wire is for the volt gauge is, or see a wiring diagram that's shows it. If anybody else has any ideas what could be going on please do share. I'm pretty sure the ground is shared through the entire cluster so I don't think it's that since all the other gauges are good. I'm pretty sure it's the gauge or the power wire. Any info on testing the gauge would help too. I'm gonna do some more searching the forums but any info you guys have would be appreciated. I'll post a couple pics below.

It's a 1995 GMC Sierra, 4.3 L. Only thing not stock is a pioneer head unit, no aftermarket speakers or amp.

[Custom/Ten]

key on engine off, sometimes bounces around a little like this. It will fluctuate when engines on and gets worse when I start adding a load. Before it started screwing up yesterday it would read 12ish in this position.

[Custom/Ten]

This is the reading at the battery at the same time, engine off. it was 12.6 when I came out this morning this after running some accessories and probing around for a few minutes.

[speedygonzales]

Why don't you try just taking a wire temporarily from the battery to back of the gauge and splice it in and see what it does. Could be the permanent solution.

[franken]

12.49V is a somewhat discharged battery which indicates a problem. A charged battery is 12.6V.
12.5V is ~85% charged. https://www.google.com/search?q=12+b...tm%3B329%3B344
I'd measure the battery voltage when the engine's running.

[Custom/Ten]

Engine running

[Custom/Ten]

This is right after engine shutdown, I believe that's called a surface charge.ill snap another pic when the surface charge wears off.

[Custom/Ten]

Just incase I was unclear ( which I usually am ,sry) my volt guage in the cluster does not match the readings I'm getting with my multimeter.

It may be worth mentioning that this all started while driving in a massive rain storm yesterday and I was kinda hoping that something just got wet and would of dried out and fixed itself by today, it didn't. Thanks for your responses so far!