Owlet

I want to hardwire the radar detector. I looked at MirrorTap, but I don't think it's possible, as it seems there are no 12V wires that switch off when ignition is switched off. There's also a wire that can be tapped into the fuse box under the rear passenger seat. I don't want to do that. What I'm trying to do is run a wire from the radar detector around the windshield and somehow tap into the same place where 12V cigarette lighter plugs in. But I don't know what I need to disassemble in order to have everything hidden. I know that's what professional installers do and it doesn't take long. It must be pretty easy to accomplish. Did anybody do it that way and can share instructions?