Under the hood on drivers side it should be mark on the outside of relay and fuse box.
The 2009 Chevrolet Silverado-3500HD is 6 ft. 7.9 in. (79.9 in.)12V front power outlet(s) wide.
The 2012 Chevrolet Silverado-3500HD is 6 ft. 8 in. (80 in.)12V front power outlet(s) wide.
The 2011 Chevrolet Silverado-3500HD is 6 ft. 8 in. (80 in.)12V front power outlet(s) wide.
The 2013 Chevrolet Silverado-3500HD is 6 ft. 8 in. (80 in.)12V front power outlet(s) wide.
The 2014 Chevrolet Silverado-3500HD is 6 ft. 8 in. (80 in.)12V front power outlet(s) wide.
The 2008 Chevrolet Silverado-3500HD is 6 ft. 7.9 in. (79.9 in.)12V front power outlet(s) wide.
The 2007 Chevrolet Silverado-3500HD is 6 ft. 7.9 in. (79.9 in.)12V front power outlet(s) wide.
The 2010 Chevrolet Silverado-3500HD is 6 ft. 7.9 in. (79.9 in.)12-Volt Power Supply (Fleet) wide.
The 2012 GMC Sierra-3500HD is 6 ft. 8 in. (80 in.)12V front power outlet(s) wide.
The 2013 GMC Sierra-3500HD is 6 ft. 8 in. (80 in.)12V front power outlet(s) wide.
The 2010 GMC Sierra-3500HD is 6 ft. 8 in. (80 in.)12 Volt Power Supply (Fleet) wide.
The 2008 GMC Sierra-3500HD is 6 ft. 8 in. (80 in.)12V front power outlet(s) wide.