You are on the right track there are two switches you should have power to one side of that switch that is wired the other wire goes to the lights in back. If you do not have power then check up hill to the fuse box you might find a fuse connection problem or a blown fuse.
here is a diagram hope it helps.
what year is your car?
one switch or both will work the tail lights. One switch is activated by the frontbrake circuit and the other is for the back brake circuit.If the fuse isn't blown put the wires on the other switch and see what happens....Now quickly to the garage
yep one wire should always have power to it.check it with the key on too just incase it has been re engineered