Originally Posted by Cole

You could try buying an after market USB lighter adaptor and gut that... Otherwise you need to delve into the world of electronics and make something up with an LM7805

Amazon.com: iFlash Dual USB Car Lighter Charger Adapter with 3A Output - fast Heavy Duty Ouput (Black): Cell Phones & Accessories

12V to 5V Converter | Circuit Diagram

How does the manufacturer of your dash cam expect you to use it in the car if they don't supply a 12V feed? Is it a GoPro or something like that with its own battery?