tail light stopped working

[stlukey]

1985 K100RT tail light stopped working. Brake light works fine.  Works fine on park setting. Fuses all good. Bulb is good. Wiring looks good, just no juice flowing when the bike is running. So, before I have to tear the fairing apart, any insight from you kind fellows? Suggestions and educated guesses welcome. Thanks. John

[Scott_]

All I can see in the wiring diagrams tells me that the wire from the tail light goes to the BMU module in the relay box. So it sounds like you are missing a signal into the BMU to energize the tail light when running. I don't have an internal diagram of the BMU.
I can suggest to check fuse's #1 and #5 to make sure it's getting power.

[Inge K.]

As the tail light functions as normal with the ignition switch in the park position, the problem must be in the
ignition switch or earlier in this circuit.

The ignition switch connector you find under the tank on the RHS, measure if you got power at the grey/blue
wire (ignition on).
If you got power on this wire....the problem is the ignition switch, which must be dismantled and cleaned.
If you don't have power on this wire....check the loop between the grey/blue and green/blue wire at the
connector for the RHS switch assy.