WARNING: This page is about a different car, the 2013 Dodge Avenger and 2013 Chrysler 200. However, it is still accessible from the selected car via links, so may be relevant.
Possible Causes
(X21) OR (X22) HORN CONTROL OUTPUT CIRCUIT SHORTED TO VOLTAGE
EXCESSIVE RESISTANCE IN THE HORN OR (Z920) GROUND CIRCUIT FOR THE HORN