In the photo I can see a red wire with an in-line fuse holder. Is that the live feed to the dash-cam? Just make sure that whatever 12V feed you take the supply from, it switches off when the car is locked otherwise the dashcam will drain the battery if left switched on.
When I wired in a towbar relay to the fuse-board, I bought a piggy-back device that plugged into where a fuse is fitted, then you plug the fuse into it so the original circuit remains protected. The piggy-back has a short flying lead which can then be connected to the wire for the new circuit, I used an insulated butt-splice crimp to join the wires.
At the time, the car was still under warranty so I didn't want to interfere with the fuse board.