Bloody Rex

When your in the Warthog in the final run cortana will tell you tto speed up because there is a gap. Go full speed but do not go strait ,go a little to the left and you will land on a structure. Get off the hog and go down on the little bridge on top of the road where you normally drive by. There should be two doors go on the one on your left and there should be the word rex in bloody letters