Don-vic

Hello from a new member.
My wife drives a 2010 Mazda 3 hatchback 2.2lt diesel model MZR-CD. I am in the process of installing a wireless reversing camera (sometimes called a backup camera) and have struck a small problem.
The front unit comprising screen and wirless receiver needs to be connected to an electrical circuit that becomes active when reverse gear is engaged (its a manual gearbox). If the connection is made to any other accessory point; the screen needs to be switched on and off manually each time it's used - not good.

Elsewhere on this and other forums, I have seen instructions to connect to a wire or terminal at the passenger footwell behind side kick board, but the descriptions vary between models. Given that there are about 20 or 30 wires to choose from, I have decided to stop and seek further advice.

Can anybody give me an exact description of the wire I should be hooking up to. Or is there a better place to go with this connection.
Some threads have referred to a fuse (no.79 I think) but that doesn't exist on this vehicle.