When I had my Model S I was able to use mirror taps in order to provide switchable 12V power from behind my rear view mirror. With the Model 3 there are more limited options to obtain switchable power.
This install I decided to give Blendmount a try. This is my first time using their mounts and I’m very impressed. This is an all metal bracket that mounts firmly to your rear view mirror. It has adjustments in every direction so you can position it just how you like it. No more suction cups falling off the window or sun visor clips out of your normal field of view. It also makes for a clean install right in line with your normal eye sight and well in reach for a quick Mute button press when needed. You could also install a remote display with mute (https://amzn.to/2SZa10H) if you want.
Next up I’ll be running a hardwire from the Valentine One to my driver’s side foot well area. Unfortunately there are no switchable 12V power sources in the rearview mirror (my normal power source). This requires a bit more effort in routing a cable and patience but shouldn’t be bad. I’ll be updating this post as I get that complete in the next few days. A video will also be posted at www.YouTube.com/LivingTesla