Customising bike, issue with new tail light...

[alantf]

Originally Posted by beefjerky

Ground is White / Black, mount this to the bulb housing.
New Red connects to original Brown.
New Black connect to original Black / White.

Afraid you've lost me! The OLD black/white (ground) to the bulb housing - fine.
New red to old brown (live feed) - fine
New black to old black/white (ground) - ? - This means that you have one live feed and two grounds. You should have two live feeds and one ground.