I was addressing the off-road question by mick238, but even so, this is a matter of state law. Looking through The National Highway Traffic Safety Administration (NHTSA) Interpretation Files we find letters from that department's Chief Counsel explaining Federal regulations concerning lights, car parts and other related items. These letters explain to the public how the NHTSA understands Federal law and regulations.
In a 1987 NHTSA letter to Mr. Richard F. Anderson, Regional Manager Phoenix Products Company, Inc. Milwaukee, WI, Chief Counsel of the NHTSA Erica Jones, states, "Once a vehicle has been sold to its initial purchaser, the safety standards no longer apply. The only restriction that pertains to it is that any modification performed by a manufacturer, dealer, distributor, or motor vehicle repair business must not render inoperative in whole or in part any device or element of design installed pursuant to a Federal motor vehicle safety standard."
In 2008 the Chief Counsel of the NHTSA, wrote to James A. Haigh, Vice President, Technical Specifications & Application Development, Transpec Worldwide, about a device they wanted to market. He explains that it is illegal to certain entities under Federal law 49 U.S.C. 30122, but not for the vehicle owner. The Chief Counsel writes, "We note that § 30122 applies only to manufacturers, dealers, distributors, and motor vehicle repair businesses. Therefore, if an electronic message board, such as the Merge Alert, is installed by a vehicle owner, without assistance from a manufacturer, distributor, dealer, or motor vehicle repair business, the owner is not violating Federal law if (s)he installs it and uses it." Anthony M. Cooke, Chief Counsel, U.S. Department of Transportation, NHTSA, 4/8/08
In a 1993 letter to Bob Dittert, Trooper, Texas Department of Public Safety, the Chief Counsel of the NHTSA answers Trooper Dittert questions about various changes that are commonly done by automobile owners. The Chief Counsel of the NHTSA responded in part by stating, "Under S108(a)(1)(A) of the Safety Act, no person shall manufacture or sell a new motor vehicle or new item of motor vehicle equipment that does not meet all applicable FMVSS's." " …The prohibition in S108(a)(1)(A) against selling complying vehicles and items of equipment does not apply to a vehicle or item of equipment after its first sale to a consumer," "... Please note that the "render inoperative" provision of section 108(a)(2)(A) does not apply to actions by individual vehicle owners."
Later in this letter and in direct response to the Trooper's question, "Concerning the installation of non-complying automotive equipment, i.e., sun screening, tail lamp 'black out' lenses, neon license plate lamps, etc., is this allowed by the owner but prohibited installation by a commercial entity?" The answer given by the Chief Counsel, "You are correct that S108(a)(2)(A) of the Safety Act regulates the modifications of only the commercial entities listed in that section of the Act, and that the Safety Act does not prohibit an individual from modifying his or her vehicle such that it no longer complies with the FMVSS's. The States may have requirements governing the modification of a vehicle by individual owners." John Womack Acting Chief Counsel, May 5, 1993
In a 1996 response to R. Wender, P.O. Box 456, Flushing, NY, the Chief Counsel of the NHTSA wrote, "Because of the potential of a flashing CHMSL to cause confusion with the pair of steady-burning stop lamps mounted lower on each side of the vehicle, it is our opinion that the installation of the aftermarket CHMSL would make the regular stop lamps partially inoperative within the meaning of the prohibition. The prohibition does not apply if the modifications to a vehicle are done by its owner, and you would not be in violation of the Federal statute if you personally installed the CHMSL. However, whether it is legal to use it depends upon the law of the state where the CHMSL is operated." Samuel J. Dubbin Chief Counsel, ref:108 d:6/6/96
In 1995, the Chief Counsel explains very clearly the NHTSA's understanding of FMVSS's and other NHTSA regulations. This response is to Mr. Donald T. Hoy, Senior Marketing Manager, Clean Air Partners. The "make inoperative" provision does not apply to individual vehicle owners who alter their own vehicles. Thus, under our requirements, individual owners may install any item of motor vehicle equipment regardless of its effect on compliance with the FMVSS's. However, NHTSA encourages vehicle owners not to degrade the safety of their vehicles. Philip R. Recht, Chief Counsel ref:303, dated:2/27/95
As recent as 2015, in an explanation to Mr. Alan Napier, of Earl Stewart Toyota in Lake Park, Florida, the NHTSA explains, " that applied to your inquiry, because of this statutory requirement, no person can legally sell or offer for sale a new vehicle that had been repaired if the new vehicle does not comply with all applicable FMVSSs. However, once a vehicle is sold and delivered to its first retail purchaser, the vehicle is no longer required by Federal law to comply with the FMVSS's." O. Kevin Vincent, Chief Counsel, Ref: 49 USC Sec. 30122, Dated: 4/17/15
All of these official responses and many….many more may be found at
http://isearch.nhtsa(dot)gov
