Dash Lights 2005 GMC
 

My dash lights on my 2005 GMC Sierra 1500 extended cab keeps going off and on, I can hit on dash and lights will come back on, I am thinking it may be a bad ground but not sure where to check the ground wire. Sometimes they will stay on for quit a while, other times only a short period of time. Also sometimes they will be dim. Sometimes I can move the ajustment wheel up and down and they will come on. Also sometimes when I hit a bump it will cause them to go off. Any suggetions on what may be causing the problem and possible fix?