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.