A latch can be movably mounted to each of the side rails for movement between an unlocked position and a locked position. A release actuator can be movably mounted to the rear header to extend adjacent both latches for movement between a non-release position and a release position. When the rear header is seated in the rear header seat and the release actuator is in the non-release position, each latch engages the rear header in the locked position to lock the rear header and side rails together. When the rear header is seated in the rear header seat and the release actuator is moved toward the release position, the release actuator engages both latches and moves the latches to the unlocked position to unlock the rear header from both of the side rails.