I hardwired my dashcam to the rearview mirror power today. Snapped a few shots before I closed it all up.
I used the EDO Tech® Ultra Compact 5V USB Direct Hardwire Adapter Kit. The kit ends in a USB port, so I also got a USB to mini-usb adapter. I did it this way so that in the future if I changed dashcams or wanted to use it as a phone charger I should just swap the USB adapter at the end.
Amazon.com: EDO Tech® Ultra Compact 5V USB Direct Hardwire Adapter Kit for Apple iPhone 5, 5S, 5C, 4, 3G, 3, HTC One, Nokia Lumia, Google Nexus, Sony Xperia, LG Optimus Smartphone Phone GPS Navigation SiriusXM Satellite Radio & Dash Cam: Cell Phones
I just soldered the leads to the two wires going to the mirror. Taped it up and put the adapters in the headliner and closed everything back up.