If you have Hadley horns like my '85 345, Component Products at 800-875-4488 has parts for them. I emailed Hadley and they said to buy from them.
My horns are powered by an air line that goes back to the tank for the rear suspension. It uses a small tank and a solenoid valve mounted under the left front fender to control air to the horns. I had to have a new valve. The valve has the wrong size fitting on the output so I have an adaptor ordered at NAPA -pick it up in the morning and then entertain the neighbors with the horns.