You will need a SPDT Toggle switch (3 prongs).
Cut the wire leading up to your stock horn.
Attach the wire going to the stock horn to one of the Side prongs.
Attach the wire leading back to the steering wheel to the Middle prong.
Attach a wire from the remaining Side prong to your shocker solenoid.
Here is a thread with a diagram. (Last post of the thread)
http://www.trainhornforums.com/showthread.php?t=1429&highlight=toggle+diagram