Okay, please tell me if I am incorrect...
To test an authentic horn, I believe all I need to do is connect 6v. to one wire and ground the other.
Yes? No? I'm full of poopy?
Put a few drops of light oil on the bearings. Make sure the commutator is clean and the brushes are in good shape. Start with the adjustment screw loosened (counter clockwise) a bit to see if the motor will turn over. Then screw in the adjustment screw one click at a time (clockwise) until you get the sound you want. One other thing to do is to take the front off and put a little grease on the ratchet wheel that causes the noise.