Here is a basic explanation on how to test and wire the brake switch on a Series III FFR 24v.
If you have a multimeter - use resistance scale to find the pair of lucar spades that go to the internal bulb , alternatively use a pair of wires from your battery and find the spades that illuminate the bulb. If you are unable to illuminate - then open the switch and change the blown bulb.
You should have two white / black wires because they are for the same circuit loop that goes from the test switch around the dual brake circuit shuttle valve & the servo vacuum loss switch and returns to the test switch. So when you have found the spades that illuminate the bulb connect the white (Ignition controlled live feed) to one of these spades and one of the black / white wires (it matters not which) to the other spade.
Next - if you have a multimeter - just check the test button contact is working OK - it should be N/O then connect the remaining black / white wire to one of the remaining spades (it matters not which) and the black wire to the final spade.