Why is horn stuck on?

The horn button on the steering wheel may have an internal short. The horn relay may be shorted. Two wires, one to the horn, may have shorted together.

Stuck contacts in the horn relay or a short in the horn switch wiring , Or the clock spring . What this video VEHICLE RELAYS Operation Diagnosis

Have someone listen under the hood for the sound of a clicking relay as someone actuates the horn ring. Remember if the horn does blow you might scare the listener!!!

If no click check for the under hood relay. If relay not activating check the horn ring. Some times adjacent relays have same pin out/part number. Swap relay and try. Many of the cars use a "clock spring" design on the wheel. Problem air bag circuit makes it difficult to service. Check to make sure all other services are working attached to the wheel (Speed control, radio,etc). If clock spring is failing eventually other circuits would be affected.

If you have key-less entry and have a horn beep confirmation try the FOB. No beep than maybe horn clock spring OK! In no horn beep confirmation try the panic on the FOB. That should make a little noise, and if nothing else you can check your lights.
Ck the fuse if OK
Hot wire horn if still no horn
Replace Horn

The wire to the horn button is a live wire and it is shorted out when you press the button. This activates the relay and blows the horn. relays normally work by power coming to a terminal through the coil and out the other terminal to earth . This connects the coil and so the contacts are pulled together to allow power from the battery though to what ever is the relay for .. So what you need is power to a coil terminal on the relay and the horn wire connected to the other terminal. This will complete the circuit when you press the horn and the relay will close and the horn will blow.. To get this it may be necessary to piggy back off the power wire ( battery terminal on relay ) to a coil terminal .

do you have the alarm option, factory?
there are 2 horns
and 1 horn relay.
if the horn relay is energized, then the inputs to it are bad.
bad input is the horn relay has 12v across is coil pins, all the time.
so unplug the horn wires. silence.
then check relay , now that its quiet. is the coil hot?
hot means 12vdc across it. not thermal hot.
the relay on the 97 can be active for 2 reasons.
1: horn button stuck on, so pull that wire.Gray-org, at column
2: BCM is active same color, C2 in 13 is 0volts. (active)
if the BCM is honking the horn, we ask why, and that be alarm.
im reading at alldata.com so can you , under antithieft alarm sections.
if the alarm is activated, (is it) and any door switch is open, the horn honks.
if the gray-org. wires is at 12vdc , then the relay is bad
shorted, main contacts inside.
if the wire is 0v, its wrong.
ill stop, and wait until you find which it is, bad relay, bad horn button
or bad BCM or bad input to BCM.

replace the horn as the coil inside it is probably shorted or the points are stuck together

Most vehicles have a horn relay, which should be in a fuse box under the hood. Toyota is usually pretty good at identifying the various relays and fuses in the fuse box. If your original horn was shorted, it very well could have welded the contacts in the horn relay together. Also an aftermarket horn could be drawing more amperage than an original equipment unit, causing the fuse to burn. This last statement is purely conjecture.

You will have to trace the wiring. There is a short somewhere before it reaches the actual horn or light. Horns pull an extreme amount of amperage. The relay may be stuck or the wiring to the switch may be bad. My bet would be a shorted switch or relay.

• The horn relay might have welded contacts (stuck in the on position). Pulling the relay should stop the horn.
  
• The wiring harness on the steering column might have a broken wire to the horn relay shorting to someplace that applies power to the relay. In this case, turning the steering wheel all the way to one side or the other may stop the horn, and it won't sound when you press the horn button.
• The horn button in the steering wheel is stuck on or shorted.

The switch (horn button on steering wheel) needs to be checked.