I haven't done a towbar for a few years but if I didn't have a wiring diagram, I would find the black wire which was always negative and tap into the wires coming from the clusters with Scotch blue clips. If I ended up with the tail light connected to the the indicator or the stop light connected to the fog light, I would simply swap them over until they were correct. Never failed for me. A bit tricky with the B-MAX though because of the lights in the tailgate.