Honda cb125 wiring issues. Tailight.

Hello everybody.
I've got a mega issue I just can't get my head around.
Basically my tailight will only work either in park, or on.
But only if I unplug the headlight (blue/white) cable from behind the headlight.
I've wired in a new aftermarket tailight.
3 cables, ground( green). Stop(green/yellow) and tailight ( brown)

Any help would be great as I can't seem to find any info anywhere!

Take your multimeter to your wires to make sure they don't short anywhere due to some rubbing in the past...