The other thing to do is just solder a new earth to each light board, and connect to a nearby earth point, I always hard solder in the light boards on mine, just saves so much hassle, do that at the same time as my brake light fog light mod, that is the rear fogs work with the brake lights, but still on their own, when switched on the normal brake lights still work, totally 100% legal and mot passable.
tabetha