So the problem is the horn relay. The stalk horn button works, the horn works (we bypassed the relay and the horn goes off when the stalk button is pressed). So I am ordering a relay today and voila, problem solved. Thanks everyone for your help and suggestions.
Bosswrench, I recently installed a new quick release MOMO steering wheel with the Pantera logo in the center. It would take a lot of work to modify it to take a horn button, and I might ruin the center logo with the Isis.