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Here is the official transportation code here in Texas:
Sec. 547.602. MIRRORS REQUIRED. A motor vehicle, including a motor vehicle used to tow another vehicle, shall be equipped with a mirror located to reflect to the operator a view of the highway for a distance of at least 200 feet from the rear of the vehicle.
Acts 1995, 74th Leg., ch. 165, Sec. 1, eff. Sept. 1, 1995.