Horn wiring question

[Trailpimp]

What are the two wires connecting to the male plug on the horn? I am assuming one is ground and the other positive?

I am trying to install my old hella supertones on the D2. Does anyone know where to locate a wiring diagram?

 

[neil30076]

Black is ground always on a british vehicle, purple/green is the + feed.
Neil

 

[jeast]

supertone help please

How do I tell which is positive and which is ground on the Supertone horn itself? Both tabs appear identical. Thank you.