After searching the internet I had to resort to scalpels and multimeters to define the pinout of a Garmin Virb action cam.
These cameras are well specced and are equipped with GPS and Ant+. The camera can be acquired from Johnny Appleseed GPS at this link.
In order to connect the camera to other devices such On Screen Display (OSD) for video transmitters on UAVs or small vehicles a slightly more efficient cable needs to be used compared to the Garmin issued version (link)
The 4-to-1 cable provides 3.5 mm stereo input for an external mic, RCA composite video output, 3.5 mm stereo audio out and mini-USB port.
The aim is to come up with a small and light cable containing only the essentials. Garmin uses a resistor connected between one of the data lines and USB Ground to determine if the camera should switch to external view mode. The connector is a Mini USB B but equipped with 10 pins. Connectors can be obtained from a variety of sources on eBay or other sites. ReadyMadeRC have them for US$0.89 (link)
Pin 1: USB Power (5v)
Pin 2: USB Data +
Pin 3: USB Data -
Pin 4: Garmin Video Enable 2.2kOhm resistor to Pin 5
Pin 5: USB Ground (Garmin Video Ground)
Pin 6: Garmin Audio Out
Pin 7: Garmin Audio Ground (Mic Input + Audio Output)
Pin 8: Garmin Mic Input channel 1
Pin 9: Garmin Mic Input channel 2
Pin 10: Garmin Video Out
After a bit of testing I now have a breakout cable about 3cm long and the diameter of the usb connector ready to connect to another external device.