The extra wire may be for the horn. In 57 the horn wire went down the hollow steering shaft and out the steering box to the horn relay. In 58 the put it on the outside the column.These are the colours for the 57 Fords I can't say the 58's are the same but I would think they are.
The first colour is the wire the second is the tracer.
Left rear turn green/orange
Right rear turn orange/blue
Stop light switch green
Flasher blue
Right front turn white/blue
Left rear turn green/white
The seventh wire should be the horn.
Gerry
[This message has been edited by Gerry (edited 01-21-2005).]