Just thought I'd share my findings on this subject in case anyone else needs to replace or wants to rebuild their alarm horn. These babies are hard to find & expensive to replace so...........
Drill out the rivets on the horn, scrape off the gasket, and pull it apart. Inside the horn there are some contacts. Run the leads of an ohm meter across the coil to ensure the coil is good. If it is bad trash the horn. If not there are a set of contacts just above the coil. These become worn and don't provide a good circuit. File them smooth with a oiece of brass shim stock. 95% of the time that will be the problem with the horn. I have tried repairing two horns already and both times that was the case. Hope this helps.
Mike