Vehicle owners are often confused regarding the effect installation and service of third party accessories has on warranty. This confusion often is the result of intentionally misleading information intended to convince you that only the manufacturer or authorized dealer should service your vehicle.
 
This practice is also known as "Tie-In Sales" Provisions. Legally tie-in sales provisions are not allowed. These provisions require an owner of the warranted product to buy an item or service from a particular company to use with the warranted product in order to be eligible to receive a remedy under the warranty. For example, in order to keep your new vehicles warranty in effect, you must use genuine parts. Failure to have scheduled maintenance performed, at your expense, by the dealership voids this warranty.
 
 
What does the law say?
 
Under the Magnuson-Moss Warranty Improvement Act, a vehicle manufacturer may not make its vehicle warranty conditional on the use of any brand of video interfaces unless the manufacturer provides the system free of charge or the Federal Trade Commission (FTC) has specifically published that only the vehicle manufacturer’s product may be used. To challenge a false claim, ask the person to put it in writing, or request the vehicle manufacturer’s video interface free of charge. If you are charged for the video interface, or they refuse to give you a written statement, there may be a violation of Federal law.

This is the actual language of the act:
 
"No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer’s using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade, or corporate name; except that the prohibition of this sub-section may be waived by the Commission if—
 
(1) the warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and
 
(2) the Commission finds that such a waiver is in the public interest.
 
The district courts of the United States shall have jurisdiction of any action brought by the Attorney General (in his capacity as such), or by the Commission by any of its attorneys designated by it for such purpose, to restrain (A) any warrantor from making a deceptive warranty with respect to a consumer product, or (B) any person from failing to comply with any requirement imposed on such person or pursuant to this chapter or from violating any prohibition contained in this chapter."
 
Complete Information on Magnuson-Moss Warranty act
https://www.wikipedia.org/wiki/Magnuson-Moss_Warranty_Act
 
 
What does this mean to consumer?
 
Purchasing and having accessories installed in your vehicle does not void your warranty unless the manufacturer has been able to convince the Federal Trade Commission that the installation and/or service will cause a malfunction to the vehicle and the restriction is in the public interest. These are difficult tests to meet, especially since the public is economically and technologically ahead through third party accessory options. If your manufacturer or dealer tells you that your warranty is void or able to be voided due to third party accessories or service, ask him for the Federal Trade Commission determination supporting the claim.
 
You may even want to call the FTC yourself.