TR6 horn help needed

["Mark S. Lepore, MD" <Sirmoog@ix.netcom.com>]

I've had horn problems on my 1971 TR6, but think I've narrowed the
problem down:
When the car is off, no key in ignition, the horn works fine.
When the key is inserted and the ignition is turned the horn doesn't
work.
I think there must be a wire or something on a wrong contact on the
ignition switch.  
If anyone can explain how the wires should be connected and what I
should look for, I'd be very appreciative.
Sirmoog
71 tr6 cincinnati