Atro van horn blow when you turn the wheel

Usually a bad clock spring under the steering wheel. Does it stop if you rotate the steering wheel? The clockspring is a mylar tape that conducts power to the steering wheel switches. It winds up and unwinds as you turn the wheel. The conductors can short or open. Or, you could have a bad horn switch, but pretty rare. Clockspring is more common.

Often times the problem is a weak horn relay or a weak horn pad on the steering wheel. You can remove the horn relay or fuse to prevent the horn from blowing until the problem is fixed.

Which fuse ? Number an amp rating .

Sounds like a problem with the contacts under the air bag.
You may have to remove the air bag to check it.

sounds like the anti theft is malfunctioning.does the horn beep once or keep beeping

This is your security system. Your van thought it was being stolen. Next time do not leave your keys in the van and lock the doors.

The contact plate in your steering wheel is getting moisture and/or contracting and making contact triggering horn

• 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.

Don't see how one would blow the fuse an not the other . The remote function is controlled by the BCM - body control module or vehicle interface . Regardless ,the module an the switch on the steering wheel both energize the horn relay ! Do you know what a relay is an how it works ?
Check the horn relay

VEHICLE RELAYS Operation Diagnosis

hoorn take the wire off hook another horn up see if it is the horn itself if not it is the horn relay or the horn button