Will installing a tesla screen or aftermarket headunit void my warranty?

NO! Installing a headunit (or any aftermarket part) in your car will not void the warranty. 

The dealer may refuse warranty service on a covered part of your car, but only if they can prove that the aftermarket part caused the covered part to fail. 

What is the Magnuson Moss Act? 

The Magnuson-Moss Warranty Act is a federal law that was enacted in 1975 to protect consumers in the United States by requiring manufacturers and sellers of consumer products to clearly and conspicuously disclose the terms and conditions of their written warranties. The Act applies to any written warranty on a consumer product that costs more than $15 and is intended to be used or sold in the United States. 

According to the Act, an automobile manufacturer cannot void your vehicle warranty due to the installation of aftermarket parts, unless the aftermarket part caused the vehicle failure, or contributed to it (15 U.S.C. 2302 (C)). This implies that the warranty cannot be “voided” if the dealer has no claim.