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Tail light drama

Mule

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Two Harbors, MN
The bike is a '77 CB750K. In the process of the build it got a new wiring harness from VintageCB750, the stock tail light & blinker assembly was removed, a cat's eye tail light from Dime City was mounted on the fender and turn signals mounted to the frame just below rear of the seat. Front headlight is the unit with built in LED turn signals.

All lights & signals work great except the tail light itself. Brake light works fine but when you hook up the tail light loop it consistently blows fuses. If I reverse brake & tail light (ie hook up the brake light to the tail light loop) then applying the brake blows the fuse.

I'm NOT an electrical wizard, we've tried all sorts of grounding combinations etc to no avail. This new light is a 3 wire rig, whereas the stock rig is 4 wire if I remember right?

At this point I'm wondering if either a 2 wire self grounding light or an LED tail light might be the solution? Any thoughts on the problem would be hugely appreciated.
 
Do you have a Multimeter? I use a cheap $5 one from Harbor Freight. After watching a couple of Youtube videos its pretty easy to figure out how it works. Use it to determine what wire is what. You can figure out what's ground, what feeding power, and continuity between different wires. Sounds like you've got some mismatches.
 
You gotta voltmeter check it to see how it is wired for sure. Could be faulty part with internal short. As well some may depend on how the rest of your system works. Some things ground right there and others carry the ground elsewhere to ground remotely. Perfect example of that would be starter solenoids or horns, they can be either way.
 
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