Just had a very interesting day. On my last run I noticed the brake lights had stopped working again. So I checked the circuit and the switch and found everything was OK. There was 12V on one of the switch terminals and the other terminal connected to the lights.
But when I bridged the switch nothing happened. But, when I
put a direct 12V to the lights terminal as a test. everything lit up.
So it seems that although there is a 12V supply on one switch
terminal when measured with a meter, once you close the switch and put
the load of the lights on the circuit there isn't enough current flow to
illuminate the lights. So I can only assume that there is some
resistance in the live wire circuit.
Now, a 'real' mechanic would have spent hours tracing the circuit
back to try and find the problem. I spent 10 mins wiring in an
alternative 'jump' supply from the ignition-live circuit to the switch.
I even included an in-line fuse.
Brakes lights now working again !