Here's my step-by step "how to", not all the tough really.

1. Split the horn in half.
From the power side, take a drill bit that is larger then the swedge and drill until it is no longer holding the two halfs together. Drill all 6 and pull it apart. Make note of the orientation of the metal diaphragm and the back halve to the trumpet. While you have the drill out, take a number #29 drill bit and drill out the trumpet side and tap it for a #8-32 machine screw.

2. Look at the inside of the magnet and you’ll see two small contacts, that’s where all the problem is. Take a piece of sand paper folded in half, open the two contacts with a screw driver then slide the paper between them and take the screw driver out. Pull the sand paper, this will clean both contacts as you do that.

3. If the cardboard gasket that is on the trumpet is bad, cut a new on from any paper stock the thickness of, lets say, the cover of your Mopar Muscle magazine.

4. Your ready to test.
Reassemble the horn, I use 3 vise grips and make sure they’re tight.

5. Screw the horn assembly together and use a cutoff wheel to trim the threads flush then use the side of the wheel to grind off to a flat the other end of the screw.

I've done a half dozen horns so far and they all work.