1977 MGB Steering column switches (wiper and turn signals) 

Being that the purple wire is always hot I would suggest that you stick to only the original push horn on the turn indicator arm. Or retro fit the earlier disk and brush horn arrangement. The spool system you have suggested to will over time fail due to its heavy use. This may cause subsequent electrical fire or constant blowing of the fuse.

What brand of wiper switch did you get? Mine needs to be replaced.