You'll need to sit the antenna cover on your roof with the car sitting level, and check the angle of the rear inward slope.
When you cut the hole for the camera to poke through, you'll need to mount the camera close to level, but pointing slightly down will be better than pointing slightly up.
Remember, that camera has a 110* Field... in all directions. You don't really need the up or down FOV, but side to side will be out of perspective if the camera is sorely unlevel.
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