Brake light stopped working

For some reason the brake light on my 74 commando has stopped working. Neither the front or rear brake switches will light the bulb. If I connect the the wires at the rear light assemble directly to a 12v battery it works fine. If I put a multimeter to the 2 connectors at the rear brake switch I get no reading. Does anyone have any clues as to where the problem might be? Thanks for the help. Ross

Ground. At the tail light. Just a guess.

Does the running light work proper?

Yes the running light works fine. It's just the brake light that does not work. It was fine yesterday.

Found the problem. 2 of the 3 wires to the rear brake light switch had been severed by the chain. Once reconnected and rerouted all is working agian. Thanks for the help.