It wont be the ground as the light would not work at all... Since it is a twin filament bulb did you check to make sure the one filament was not burnt out? Use a VOM to see if there is power to that light with someone stepping on the brake.. if not then check the 7 way plug to see if there is power to it for that light.. Since you have not told us what truck or trailer you have it is hard to guess past that..
2004 Chev Silverado Duramax optioned past the max. 2009 Jayco Eagle 308 RLS 765 watts of solar, 6-6 volt batteries (696 amp hour), 2000 watt (4000 surge) whole house inverter.
140 days boondocking in 2016
211/2015, 196/14, 247/13, 193/12