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I'm really getting into the weeds of knowing this bike now and found the rear brake light failed last week, the front brake is fine so a long story cut short is the pressure switch is offering 0.0 ohms between peddle pressed/released = Bad switch.
Making a direct connection with both wires lights the bulb so that's good.

Decided to leave the pressure switch where it is and connect the two brake light wires to a micro switch on the brake peddle.

Strange thing is I only measured 4.2v between the two wires and expected a 12v reading, is this something to do with the Running Wire magic Black box???

RocketSteve, you won't see 12v. "across" those two switch wires, is that what you are saying ? You are reading an "in series" voltage between a source (12vdc) through a load (light) then to ground. Craig H.

Thanks Craig, I was expecting a 12v reading between the two switch wires but you've explained the set-up so that's good for me.

That is a very common failure with Harleys too

Job now done and back to full function.