They have to prove the problem was caused by the installation / part.......
Even easier to understand link
http://autos.aol.com/article/warrant...rmarket-parts/
"This means that, under the provisions of the Magnuson-Moss Warranty Act of 1975, an automotive dealership/carmaker cannot void your warranty because your vehicle has been modified with aftermarket parts. They (the manufacturers) have to prove that the failure was the direct result of the installed aftermarket part. Unfortunately, too many folks have gone to a dealer to have warranty service performed on their modified vehicle only to have the dealer refuse to cover the defective items. The dealer usually states, that because of the aftermarket parts installed, the warranty is void (without even attempting to determine whether or not the aftermarket part caused the problem). This is illegal...period."