Air horns I need to know can a set of air horns be hooked up to the OEM horn system.

No Vince

The most likely is that someone tried to break into it and it is the alarm. Horn pads don't self activate. I guess it might be a faulty relay?

https://www.google.com/search?q=2000+Buick+Century+Custom+horn+went+off+on+its+own

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Do you have an air bag light lit on the instrument cluster ? Do you have C.C. button's on the steering wheel , do they work ? If the alarm fob (as u call it ) Can make the horns sound , the relay is sending B+ voltage to the horns Don't know how your testing.
System Description
The horn system consists of the following components:
• Horn fuse
• Horn relay
• Horn switch
• Horn(s)
• Body Control Module (BCM)
System Operation
The vehicle horns are activated under the following conditions:
• The horn switch is depressed.
• The body control module (BCM) commands the horns on. The BCM commands the horns on under either of the following conditions:
- When the content theft deterrent system detects a vehicle intrusion. For further information, refer to Content Theft Deterrent (CTD) Description and Operation in Theft Deterrent.
- When the panic button is depressed on the remote control door lock transmitter. For further information, refer to Keyless Entry System Description and Operation in Keyless Entry.
- When the OnStar® system is used to sound the horns, if equipped. For further information, refer to OnStar Description and Operation in Cellular Communications.
- When the keyless entry system is used to lock the vehicle, a horn chirp may sound to notify the driver that the vehicle has been locked. The notification feature may be enabled or disabled through personalization. For further information, refer to Keyless Entry System Description and Operation in Keyless Entry.

Circuit Operation
Battery positive voltage is applied at all times to the horn relay coil and the horn relay switch. Pressing the horn switch applies ground to the horn relay control circuit. The body control module BCM may also apply ground to the horn relay control circuit as described above. When the horn relay control circuit is grounded, the horn relay is energized and battery positive voltage is applied to the horn(s) through the horn control circuit. The horn(s) sound as long as ground is applied to the horn relay control circuit.

If using the driver information center/dic, press and release info button until tire learn message is indicated.Depress and hold set/reset button until two horn chirps are sounded indicating learn mode has been activated, front left turn signal will be illuminated. Starting at the left front tire, increase/decrease the air pressure for 8 to 10 seconds and wait for the horn chirp. It may take 30 seconds for the horn to chirp. When horn chirp has sounded the sensor information is learned, then the turn signal for next location will be illuminated and so on. When all sensors have learned the horn will double chirp. Turn key to off position to exit learn mode. Adjust all tire pressures to recommended pressures. System will complete calibration process as vehicle is driven.
Knowing the model would help, the info above covers several chevy models, but, not all.

possible blown fuse or defected horn

Un plug horn connect horn to another 12 volt power source, if it works then the problem is the horn button located in the steering wheel assembley

YOU NEED AIR BAG CODE SCANNER TO RESET LIGHT/HORN CHECK HORN RELAY /FUSES/WIRING/HORNS CONTACKS IN STEERING WEEL

If you have a 12 volt test light you can pull the wire off the horn and hook one end of the test light to ground and the other to the wire for the horn then have someone press the horn button down if the light comes on then the wireing system is ok and it's the horn froze up, if it does not come on try replaceing the horn relay with a relay that is the same I believe the head light and AC relay are the same so try switching and see if the light comes on, if not and you have checked the fuse for the horn then it may be in the steering wheel where the problem is. You can test the horn by applying a wire to the positive side of the battery and touching it to the horn spade connector, if it beeps then you know it's ok and you have a problem in the steering wheel. I will stop at this point because working on the steering wheel can be dangrous unless you unplug the battery first, the reason is you could set off the air bag by accident and this would take another page of instructions to do it safely, if the air bag blows it can kill you. Hope this helps and get back to me if these test prove that the horn is good and you need to check the steering wheel assembly. Thanks for using fixya.

take a test light and hocke it up in wire for horn under car ground test light.have someone push horn if it does not light work your way back fuse horn button etc it allso a horn realy

The horn for the alarm is located beside the wiper motor assembly. You'll need to remove the wiper arm assy so you can remove the cowl panel and access the electric horn.

It might be the switch in the steering wheel, to find out for sure, take a piece of wire long enough to reach the horn from the battery,
hook one end to the positive terminal of battery, touch the wire connection at the horn with the other end, if it blows, the switch in the steering wheel is bad. ( be careful not to touch any metal parts with the wire, you will get sparks).