Thanks for the info. Here is what I found:
S220.127.116.11 (a) Each high-mounted stop lamp installed in or on a vehicle subject to S18.104.22.168(a) shall be located as follows:
(1) With its center at any place on the vertical centerline of the vehicle, including the glazing, as the vehicle is viewed from the rear.
(2) If the lamp is mounted below the rear window, no portion of the lens shall be lower than 6 inches below the rear window on convertibles, or 3 inches on other passenger cars.
(3) If the lamp is mounted inside the vehicle, means shall be provided to minimize reflections from the light of the lamp upon the rear window glazing that might be visible to the driver when viewed directly, or indirectly in the rearview mirror.
(b) The high-mounted stop lamps installed in or on a vehicle subject to S22.214.171.124(b) shall be located at the same height, with one vertical edge of each lamp on the vertical edge of the body section nearest the vertical centerline.
S5.3.2Except as provided in S126.96.36.199 through S188.8.131.52 and in paragraphs S5.7 and S7, each vehicle must conform to the following requirements:
(a) Each lamp and reflective device must be installed in a location where it complies with all applicable photometric requirements and visibility requirements, with all obstructions (e.g., mirrors, snow plows, wrecker booms, backhoes, and winches) installed on the vehicle.
(b) A manufacturer must certify compliance of each lamp to one of the following visibility requirement options, and it may not thereafter choose a different option for that vehicle:
(1) When a vehicle is equipped with any lamp listed in Figure 19 of this standard, each such lamp must provide not less than 12.5 square centimeters of unobstructed effective projected luminous lens area in any direction throughout the pattern defined by the corner points specified in Figure 19 for each such lamp; or
(2) When a vehicle is equipped with any lamp listed in Figure 20 of this standard, each such lamp must provide a luminous intensity not less than that specified in Figure 20 in any direction throughout the pattern defined by the corner points specified in Figure 20 for each such lamp. The luminous intensity must be measured in accordance with the photometry test requirements of the applicable SAE Standards and Recommended Practices incorporated by reference or subreference in this standard.
(c) A high mounted stop lamp must have a signal visible to the rear through a horizontal angle from 45 degrees to the left to 45 degrees to the right of the longitudinal axis of the vehicle.
(d) High mounted stop lamps required to comply with S184.108.40.206(b) must together have a signal to the rear as specified in S5.3.2(c).
So if I'm reading that correctly, aside from being in the center and higher than the other two brake lights, the CHMSL also needs to be visible at a 45 degree angle from each side, which I assume would prohibit mounting behind the tire?