I think you can tap it off the same place overhead location as in the 3, but my Tesla Mobile Service tech said not to tap it from there because the car will think there's a phantom drain. He said don't go directly to the smart battery either because of the same reason. He said it was ok to use the cigarette plug, which I think goes to sleep depending on Sentry Mode status, but that shouldn't matter with radar detector. I used the cigarette plug in the center console. I followed Erik's dashcam install link and routed power to the center console plug the way he did. I got a splitter for dashcam and future V1 install. The steps are the same for 3 and Y.
I've also included the mirror/base difference between the Y and 3.