Yikes, be VERY careful! If you've never had the pleasure of being around an air bag deploying, take it from me: You don't want that.
Disconnect both battery cables. Let the vehicle sit for a while (10+ minutes just for extra caution) and then the system is ready for you to work on.
I'm quite sketchy on the used air bag though; if there's any short involved in that unknown entity, you may get more than you bargained for when you reconnect that battery...
Also - did you verify the failure is the horn pad itself? This is not a known issue for this vehicle -- it is a known issue for the Grand Marquis and Crown Victoria and Town Car -- different air bag.