A pickup truck including a cargo box having a truck bed and a rear sill adjacent the rear of the truck bed. The sill attaches beneath the truck bed. The sill includes a drain trough on its upper surface facing the underside of the truck bed. The drain trough includes a groove extending along the length of the sill. A plurality of apertures is formed in the sill within the groove. The apertures are formed toward an inner surface of a side panel of the sill. The drain trough irrigates liquid (e.g., e-coating paint) that resides between the truck bed and the sill to collect in the groove and exit the groove via the apertures, enabling the liquid to drain down the inner surface instead of an exterior visible surface of the sill.