so ya electrical can be frustrating?
sometimes it can be simple as bad connections?
like JR said grounds are very important and people tend not look at that?
1) remove cables from batteries +/- and clean, scrap, wirebrush the **** out of the terminal post & inside of connectors. if you see a bunch of buildup were the wire goes into the connector they may be bad. sometimes you can get away with pouring coca-cola on it temporarily. clean everything up real good!
you would be surprised how many times my crew would call and say the equipment wont start even jumping it. i would make them clean the connections up then everything was good to go.
2) i use a little motorcycle battery, test light & some small wiring with clips on the end like little jumper cables. it comes in handy when tracking down gremlins & electrical issues. you can easily check your gauge by hooking up the little battery & little cables directly to it. depending on the gauge you shouldnt have to turn the bus on or anything. but you will have to get behind the gauge/dash to hook up the baby cables directly to it.
3) check all the ground wires behind the dash or any where for that matter. again remove it wirebrush it and put it back. some times i add additional ground wires in various places just because of how important they can be and usually dont hurt to have too many....
4) from there ya just gatta start testing things with process of elimination but it does take some knowledge. when i got my bus i got rid of all the extra wireing that didnt belong. i put everything back OEM ford & thomas. then all of a sudden my brake lights started working again! i never could figure out what the root cause was? but everything has been working perfect ever since! so keep in mind any wiring you come across that doesnt look OEM may also be a good place to start looking. but be carefull dont start yanking stuff out unless you know what your doing LOL could make thinds worse.
anyway i hope i didnt throw too much at ya?
be patient and my moto is " take two & think things through "
i hope this helps.
enjoy the build!