WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Body Control Module (BCM)
Scan Tool Output Control
Additional Menu Selection(s)
Description
Horn
Miscellaneous Test
The BCM Energizes the horn relay for 5 seconds when you select On or until you select Off.