Put the old wheel back on and test the horn - that will give you a better idea of whether there is a fault in the horn push area, or whether you should be looking further afield. Modern (OE i.e. cheap) horns don't seem to last too long...
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly '09 Gen2 MINI Cooper ragtop '90 LR 90 SW