Check your wiring loom and the wiring diagram. I got a similar tail light from OTR and found that the colour of the wires didn't match with their description.

I ended up having the brake light always on and the licence plate illumination responding to the brake lever.

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Yamaha XT660X '08' Blue | DNA Stage 1 | MTC 2 into 1 Carbon Oval | OTR Tail Tidy | Engine Painted Black |