My rearview mirror doesn't have a power source so I run mine through the top headliner, down the passengers A pillar, across my glove box and into the cubby hole under my radio and connected to the cigarette adapter.
Old setup where I had coiled cable being visible
My mount is the older version. I reviewed it for BlendMount and its posted on their site somewhere.
You can tap into the rearview mirror for power. Luckily, Valentine sells super long phone cords. Running wires to rearview mirror can be clean install as well.
I don't like to hardwire stuff and mess with the wires unless I really have to.