Voltmeter needle registering upside down and leaks around top cab lights on 2008 GMC Sierra 2500 HD

My voltmeter needle is no longer in the range of values on top(from 10 to 2 oclock) but registering now from 8 to 5 oclock. During heavy rain, water is coming in cab probably from the top running lights

Asked by for the 2008 GMC Sierra 2500 HD