Follow the Path, my son.....
Okay, I dug out the FSM and laboriously followed and labeled the flow of power for the horn circuit. You better appreciate it, it took me half an hour. Power starts at the battery with a red wire passing through the fuse box, through the white steering column connector, and to the ignition switch. The wire then turns black, goes back through the white connector, and passes through a fuse in the fuse box. The wire then turns black/yellow and passes through both horns. The wire then turns brown, goes through the white connector a 3rd time, then the horn button, and then to ground.
So, the first thing to do is to get your voltmeter out to look for power at every place where there is a wire connection, all the way to the white steering column connector. Now you follow the "Divide and Conquer" troubleshooting procedure. Power starts at the battery and goes all the way to the horn button through multiple connections and then to steering column ground, any one of which could have a disconnection. The red wire powers the ignition switch and you know you have power to there, so no need to check that....yet. "Divide and conquer" means to go to the easiest end of the line point to check for power and see if it is there, if it's not there, go halfway back, if not there, go halfway back again. This is the divide part. As you may or may not know, power switching in a car is done by "removing ground". What this means is that most un-ignition-switched devices in your car have power going to them mostly all the time and you "switch" them with some sort of button. Brakes, horns, dome light, etc. are always live and they get activated or GROUNDED when you step on the brake, push the horn button, or open the door, etc. Therefore, YOU SHOULD BE ABLE TO DETECT POWER AT EVERY WIRE CONNECTION FROM THE BATTERY ALL THE WAY TO WHERE THE BROWN WIRE COMES OUT OF THE WHITE STEERING COLUMN CONNECTOR. The only way to detect power to the horn button is to do some horn button removal and mess around inside the steering column, so let's not do that yet. Because you are checking power to ground and your voltmeter is measuring to ground, every time you go to a "test point' that has power you should hear the horns sound. If you detect power where the brown wire enters/exits the white connector and the horns sound, then you know that all the wire connections from the battery to the brown wires at the white connector are good, no need to check them. That leaves just the wire connection to the horn button and the steering column ground.
Get the voltmeter out and tell us what you find.
