Ya'll are over complicating this!
Just get a car hardwire kit from joovuu.com, think it's 10 quid, and wire the neg into the car and pos into the interior light. You can choose a type of USB connector (mini/micro) on the site to match your cam.
If you really want full stealth:
- Mobius dashcam with wide angle lens, 8inch lens extension and supercapacitor battery replacement.
Two screws pop open the mobius, pull out wire for battery, remove it, and replace with supercapacitor.
Then lift lever at end of lens ribbon, remove lens replace with wire from lens extension. Plug lens into end of lens extension.
Now you can hide the wiring kit, and mobius into your roof trim and just have the little lens poking out.