30 years ago, when I lived on Woodway Drive, just off the 610 Loop, I would have gone over to your house and we would've had this done in about 10 minutes.
Okay, here's what you're looking at, horn relay-wise...forgive my crude drawing.
Simply put, to fire a relay you need positive and negative power applied to the relay coil, in this case - positive at term 86 and negative at term 85. The squiggly line is the coil.
The positive comes from the #2 fuse to term 30 to term 86. 30 to 86 is a jumper wire which may be inside the plug or the relay. That half of the circuit is alway hot.
The negative comes from chassis ground to the horn button switch to term 85. The circuit is completed when the horn button is pushed - that causes the relay to fire.
Inside the relay are 3 more terminals; 30-which is common, 87A-which is normally open, and 87-which is normally closed. "Open" is open circuit. "Closed" is closed circuit. There's a wiper connected to 30 which goes to 87A when the relay is in the relaxed position and to 87 when the relay is energized. When the relay is energized (+) power goes from 30 to 87 to the horns. The horns also have ground wires (-).
Set your multimeter to DCV, probably the 200 range.
- With the black probe touching ground, touch the red probe to 30 & 86. You should read 12VDC at both terms of the plug.
- With the red probe on the (+) side of the battery, touch the black probe to 85. You should read 0VDC. Do that again with the horn button pushed. You should read 12VDC.
If the horn is always on, you could have a bad rubber collar on the steering wheel, a bad horn switch on the steering column, or the wires could be crossed between 87 & 87A.
Good luck...