No car dealer can void your warranty on your vehicle. In fact, most car dealers sublet their aftermarket accessory work to specialty retailers. So why would it be okay for them to do it, but not okay for you.
Firstly, car dealers want to use their preferred vendors. Most of the time it is someone they trust. Secondly, it gives them a chance to add some profit to the sale of their car.
Some instances a car dealer can void your warranty. But let’s be clear, they cannot void the warranty on your entire car. For example, you go to a specialty shop and get aftermarket fog lights installed. Two weeks after installing you find your headlight switch no longer works.
Book in an appointment at your car dealer and they find the specialty shop wired in something wrong. Leaving you with a bill when you thought you had warranty. The car dealer replaces the headlight switch, and you have full warranty again.
If you get aftermarket work done. I recommend reaching out to that specialty shop before bringing it in to a mechanic, or car dealer. Mistakes can happen, fuses sometimes blow, after all we are human.
Most professional dash cam installation shops have technical information on your vehicle and will have limited challenges. Think twice before trusting the random guy on Kijiji, or Facebook marketplace that will do it for cash.