[Beezer]

My schematic is for an 88 but the colors should be the same. Green wire is ground. Green/Yellow is switched +12 and us common to both switches. There will be a disconnect probably under the seat and it's a safe bet a bullet connector is loose or pulled out there. The only other disconnects are for the switches themselves and they are probably good since you are getting an indicator at the gauge.

 

[Beezer]

I do not trust my eye sight when checking fuses. I always use a multi-meter to check the fuses to see if they have a open circuit. I also put the positive lead in the fuse block and ground the negative lead too check for voltage and amps in the block. Make sure you have power.

Years ago I spent hours troubleshooting a very odd automotive problem in which a spade-type fuse had an ever so tiny 'fault line' in the filament. It was not quite open, not quite a full connection. It was a resistor of sorts. Forehead slapper. I've used an ohm meter from then on.

 

[Beezer]

I have been able to determine that both filaments on the tail light are in fact burning constantly without either of the brakes being depressed.

Or a maladjusted switch, stuck switch, or like was said, pinched wire.

 

[Beezer]

Ruled out since the dummy light *only* lights when either lever is activated. If a switch were always on, the dummy light would also be always on.
Forgot about that. Yep. Gotta be a pinch or touching bullets.
Sometimes those wires inside the socket get mishandled and their insulation becomes receded, exposing the conductors. Then they can twist a little, cross and touch.