Ok, I'm changing my position. Tesla is not prevented from mounting a third brake light (if under 80" wide) on the tailgate due to it being movable. They would still need to meet the other lamp requirements.
This may also extend to the identification lamps.
This may also extend to the identification lamps.
1985-03.1 | NHTSAThe assumption underlying the agency's 1980 letter was that a defect could exist if all a vehicle's stop lamps and taillamps were mounted on the decklid, where their signals could be unobserved or obscured if the lid were in any position other than closed. The center high-mount stop lamp, on the other hand, while an item of required equipment, is nevertheless a supplementary stop lamp. Even if the deck, hatch, or tailgate upon which it is mounted should be open, following drivers may still observe the signals of the primary stop lamps remaining on the body.