roof marker lights on 2003 Chevrolet Silverado 1500 HD

How do i turn on the roof marker lights? My son recently bought this truck and there is no switch to be found. Also what would the fuse be named?

The roof marker lights on your 2003 Chevrolet Silverado 1500 HD are turned on and off with the park lights, using the headlight switch. The fuse for the roof mounted lights is in the under hood fuse block and is labeled INT PARK, it is a 10A fuse. If the fuse is good I would suspect a wiring issue inside the cap as the cause of your problem.