i think you hooked up one of the wires wrong, black to black but i think your other wire from the box should go to the yellow on the bike...
reason being one of the wires is ground like you found out, one of the other is the 'running lamp' on the dual filament tail lamp, the other is the 'brake light' filament . so you've hooked up the power lead from the topbox not to the brake light but to the running lamp side of the bulb ... disconnect that lead and put it on the yellow, retape the wire on the blue ...
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