WRONG aftermarket parts ONLY void the warranty IF the issue that is being claimed if the warranty issue was affected by the after market parts. So if you install exhaust, and your power windows quick working, warranty will cover your window. If you install cat back exhaust and a head gasket blows, or a bearing gets spun, warranty will cover it. If you install cat back exhaust, and 02 sensors show that it went lean long term and the ECU coldn't compensate the manufacturer MAY have an arguement that lets them void the warranty.
Of course manufacturer will try to void the warranty for anything, but that doesn't give them the right to. They have to prove that the aftermarket part caused the issue in order for them to void that specific issue.
Even if the exhaust does cause a lean condition and destroy the engine, warranty still covers all of the power windows, gauge cluster, and all the other stuff that is unreleated to the exhaust.
Same thing with bike racks, it will void the warranty IF the issue being claimed has to do with the rack. So say the driver side door starts leaking. they might be able to make the arguement that you put too much weight on the top of the car because of your roof rack. But if your engine blows they can't blame that on a roof rack and void the warranty.