99 Camry horn honks by itself
 

The steering wheel area contains the horn switch and wiring. Grounding of the horn wire will cause the horn to work. The wiring or horn switch could be defective causing the switch or wire to ground when the steering wheel is turned.

To access the switch and wiring the airbag needs to be removed thus might leave this to a shop.

If you still want to proceed the airbag is held in by two torx screws, one on each side of the airbag hidden by small covers.

One the bag is free carefully disconnect the air bag wiring harness, lay bag assy on any surfaces with the front face oriented up.

Before doing ANY work disconnect the battery for at least 90 seconds to disarm airbag system and DO NOT reconnect battery until the bag wiring is reconnected and bag reinstalled. If you need to drive and reconnect the battery then turn on ign key when the bag wiring is disconnected, the airbag system will self test then fail then turn off triggering fault codes and need to be reset once the bag is reconnected.

Unless you use the horn a lot until fixing the problem remove the HORN fuse located in one of the plastic fuse/relay boxes on the left side of the engine compartment.

Thanks, that makes perfect sense.