I have brought some horns back from the dead by spraying a good amount of WD-40 into the mouth of the horn and then rotating it so that the puddle follows the spiral to the center. Then I let it sit awhile. Then I reverse the process to get as much WD-40 out as I can before applying 12v. If that doesn't work I give the horn a few LIGHT taps with a hammer to see if I can free the diaphragm.

I don't recommend buying old horns from eBay unless the seller says explicitly that they work. Lots of dead horns on eBay.