COWORKER WAS BACKING UP F-350 PICKUP WITH CAMPER TO LOAD ANFO BAGS FROM TRAILER. EE WAS GUIDING THE DRIVER WITH A FLASHLIGHT. AS THE DRIVER WAS BACKING UP, EE MOTIONED WITH HIS FLASHLIGHT TO STOP. THE DRIVER KEPT ON BACKING UP, PINNING EE FOR A SECOND BETWEEN THE CAMPER DOOR OF THE F-350 AND THE TRAILER. WHEN THE DRIVER HEARD EE, HE IMMEDIATELY PULLED FORWARD.