this is tough.... teaching wiring 101 this way wont get it very far...
as you may not be as full of electrical knowledge as it will take to do what you are attempting to do, (according to the questions you're asking) you should read up on how electronic gauges should be connected in to the system.... then take the thing on step at a time...
the gauges are probably hooked up there because someone was too lazy to do it correctly.
the gauges need power, but the gauge operates on a variable ground resistance thru the sending unit.... so someone connected to the closest power source, but thats only a guess, as I am not there to actually see it....
im not sure what alternator you have and/or how many wires it would normally need. some use one wire, some use two, some three... and more depending on the type of alternator and accessories you have connected into the system.
and im not even sure if you are hooking up a charger or an external regulator, as they are very different... you called it a charger, but chargers are the simplest things ever to hook up. there should be clear instructions available on the internet, if you dont have them in your hand already...
but dont feel too bad, as when even the best electrical guy has to work on a system that has somehow been morphed into a spider web of unknown wires, its a pain in the AS* that takes time to figure out... a GOOD tester is your best friend when trying to figure out an unknown wiring nightmare. something like
this you can then do about any type of electrical test or wire trace you will ever need to to on a 12 or 24 volt system... of course, thats AFTER you learn how to operate all the functions of the tester.
I have had one for a couple of years, and use it often, but STILL havent used all the functions it has available....