I cringe when I see the kids in these aftermarket shops cutting into a wiring harness. I would NOT add any aftermarket electronics without fabricating a wiring harness that would plug into existing connectors. I WOULD NOT cut or allow anyone else to cut a single wire anywhere on a factory wiring harness.
What is the problem with using the key? It cant' take more than 3 seconds to lock the door and drop the key in your pocket.
I have seen a number of cars over the years really butchered with aftermarket electronics installations. I even saw a car burn once that turned out to be directly related to electrical installation incompetency.
Good luck with whatever you decide to do,