I'm starting to think its an epidemic..

on your car's voltage regulator there is two wires,black is hot with key on,the green wire(field wire) goes to the alternator and plugs on to it,this wire should energize when the engine is running,then there is the large wire that goes to the alternator this should have power at all times.it doesn't sound like the amp gauge is bad or unhooked unless they ran it thru the aftermarket gauge because it starts and runs so I don't feel the problem is with that part of it.look at the alternator and see if it has one or two plug on connections and a larger bolt on connector,if it has two plug on connectors and one larger bolt on connector you will need to run a ground wire from one of those field wire connections to a ground and then the charging system will work.if you have power coming into the voltage regulator but not coming out to the green(field wire then the points inside the regulator may need to be cleaned or the lil wires inside may have gotten burn't thru.hope this is helpful.