Fitting High Level Brake Light to Ford Fiesta 1.25 Ghia?

I'm in the process of fitting a high level brake light in the rear window of my Ford Fiesta 1.25 Ghia. However, I can't identify which wire in the tail light cluster block I need to connect the high level brake light to. I tried the purple and black wires, but couldn't get the high level to illuminate. I've taken a spur off of an earth terminal within the boot area to earth the high level. Any help would be gratefully received.