After using this set up for some time. I have realized that the socket in the trunk has power when the car is charging and also when it's not plugged in but preconditioning.
For those that do not know this yet, the rear-view mirror compass receives 12-13 volts DC only when the car is powered up.
If you want to install a DashCam I suggest you use the hardwire DC-DC converter that SpyTec sells.
I think it costs less than $8
This converts 12-24 volts DC to a clean 5 volts that most dash cams require and it draws minimal current.
It is small enough to fit in the mirror mount if you trim off any of the extra long wire.
I have quite a bit of electrical experience. I cannot be responsible for anything that you attempt to your vehicle. You could damage systems especially if you have little electrical experience or do not put safety first. Always put a properly rated fuse in your circuit!
The wires you need to look at are +Green and -Brown.
This is not a license to put high current devices on that circuit.
Use common sense. Anything on that circuit should draw a few hundred milli-amps only!