yeah you need the earth plate etc as tobz said along with your existing wire to get horn to work, providing you got them with the wheel it should work, there only to wires to hook up??
the 2 wheels i got in my bug both had the plates missing, this was a right arse!!!, in the end i put a big yellow push button on the dash labelled 'HORN' and the friggin mot tester still missed it.... doh...
Gaz