If you don't have the manual, you can often find a PDF version online through a Chevy parts website or a general auto repair site. Search for "2006 Chevy Express owner's manual PDF."
There are typically two fuse boxes in a Chevy Express: one under the hood and one inside the vehicle, often under the dash on the driver's side. The lighter fuse is *usually* in the interior fuse box. Look for a diagram showing the fuses, and usually the legend will note which fuse is for the power outlet.