Aftermarket will not void any aspect of your warranty unless the aftermarket product can be shown to directly cause any problems you are having. For example, the BMW dealer cannot deny you coverage of some electrical problem in your car simply because you have an aftermarket camera. He'd have to show that the camera caused the electric problem.
The flipside is that you can still be denied warranty coverage for OEM parts, unless it is a product that is installed by a BMW dealer under normal procedures. So, for example, if you are an E9x owner and you order a OEM BMW backup camera from a European parts supplier and then install it in the states (where the camera is not a dealer-installed option), you won't necessarily be protected.
E91 Montego Blue gray leather burl walnut , Idrive, PDC, Logic7, CA, Sirius, xenons, 6FL, ZPP, heated seats, 17" tires.
4, 8, 15, 16, 23, 42