If you're joining us from Ranger5G, then you may already have an account here! As long as you were registered on Ranger5G as of March 27, 2020 or earlier, then you can simply login here with the same username and password.
It's not very exciting reading, but here are the FMVSS regulations for the CHMSL locations:
S5.3.1.8 (a) Each high-mounted stop lamp installed in or on a vehicle subject to S5.1.1.27(a) shall be located as follows:
(1) With its center at any place on the vertical centerline of the vehicle...