Can't get the horns on my 66 convertible to blow. They are the original ones from the factory. I've checked the electrical and it seems OK. Do any of you know what else I can check and where I might be able to purchase new horns if I need to? I've checked a few of the larger Mustang parts dealers and they don't have them. I've owned the car for 38 years and never had a horn problem before. Thanks!!

 

When you say you checked the electrical what does that mean. 12Volts at horn when the button is pressed? 12Volts on hot side of button at steering wheel. 12 volts at yellow wire on light switch? Brake lights work? Light switch internal circuit beaker ok?
That is the order I would test.
If you do have 12 volts at the horn when the button is pressed, does the horn have a good ground connection? Try some jumpers directly from the battery to the horn. No noise bad horn.

 

Is there anything that can be done to restore those old horns?

I have a couple sets laying about that sound very anemic when I put 12v to them but haven't brought myself to throwing them out.

 

You need to take them apart and file the contact . Take apart on old horns can be easy if bolted together or hard if crimped. Inside is a flexible metal diaphragm,with a center contact, and an electromagnet coil with a contact. One side of the coil goes to an insulated terminal on the housing for the 12 Volts. The other side of the coil goes to the coil contact. When the diaphragm is at rest the contact on the diaphragm touches the contact on the coil. The diaphragm is connected to the housing and therefore to ground. When you put 12 volts on the terminal, power goes through the coil, and contacts, to ground energizing the coil. The magnetic field moves the diaphragm so that the contacts separate d-energizing the coil. The note generated by the vibration depends on the size and stiffness of the diaphragm.
Corrosion of the diaphragm and the contacts make the horn weak. The price of a new horn is so low that fixing an old one is really not an option. Donate the old ones to you trash man on his next weekly pickup.
The old ahooga horns were different. They had a motor which rotated a bumpy disk against a diaphragm. They were were much more expensive and easier to fix.