2012 Chevy Express 3500. 1st sign of problem was noticed when trailer lights did not work. A few minutes later I lost all electrical power with the key in the on position, but could hear a clicking noise from inside the fuse box under the hood. When I turned the key to the off position, clicking stopped. Now dash lights up when key is turned on, but lose all electrical power when key is turned to start.

 

start by checking the battery...have it load tested. check the battery connections and make sure the cables are clean and tight

 

Short somewhere. Pull the fuses for the trailer and rear lights on van.....