Rear light cluster issue

[aquilaalba]

Hi guys, more pestering - this time electrical - thought I'd ask before breaking an open door... I've got no tail lights on when switching them on - front works fine, nothing on the back. When I press any of the brakes all the lights switch on, brake, tail light and numberplate light - all lit up when I press front or rear brake (this wasn't happening with front brake, but the cables were disconnected from the switch). I've attached a photo of a wire cut - it is (I think) for the double light mod - could this be the culprit? It was bastardised a bit, so can expect anything here I guess...