As said before, get a circuit tester. The type I have is very low tech, a screwdriver type thing containing a bulb with a cable on the end. Put the cable on an earth point and using the point on the tester poke through the wire casing. No light means no current.
So in the case of the brake switch, in situ, one wire going in will be live, the other will only be live once the switch is pressed and the circuit made. Put the tester on the supply wire first to check that is live. Assuming it is, operate the switch and then the out wire should light the tester up, if it does not its the switch. If the tester does light up, follow the path of the wire (sometimes easier said), towards the back of the car testing the wire every so often, once the tester fails to light up you will be able to narrow down the point at which the circuit fails. Whole wires rarely fail, the break will be at a connection, or where something has pinched, eg folding seat the wire breaking the circuit.
The switch itself, if the cause of the fault IIRC is adjustable, if trashed the price is £12+ vat of my parts disc. Tester is about £15 from Halfords. (Xmas prezzie to self and more useful than socks
I do not think the hatch will be a factor as the wires do not go in the hatch, the brakes are on the main body, its the fogs/reverse in the hatch.
Hope this helps