Legally, a vehicle manufacturer cannot void the warranty on a vehicle due to an aftermarket part unless they can prove that the aftermarket part caused or contributed to the failure in the vehicle (per the Magnuson Moss Warranty Act (15 U.S.C. 2302(C))
see
http://www.granatellimotorsports.com/magnusonmoss.htm
http://www.enjoythedrive.com/content/?id=7253
If a dealership denies warranty coverage, they should be willing to do so in writing. Have the dealer describe the failure which is causing your problem AND how the dealer believes the aftermarket product installed is responsible for the problem.
Refer to
http://www.semasan.com/main/main.aspx?id=60128 for information regarding warranty being denied