EE was going to work on Cannon Face Drill. Nose cone came loose from hammer 14' up face. EE used F450 flatbed serv truck to stand on to remove nose cone from drill steel. He was standing on the toolbox (7'L x 3'W x 4'H). Truck had been outside (36deg). Truck brought UG (68deg), caused toolbox to sweat. EE slipped on wet toolbox, lost balance, jumped 4' down to bed of truck.