Security Horn Relay
WARNING: This page is about a different car, the 2002 Subaru Impreza. However, it is still accessible from the selected car via links, so may be relevant.
Measure security horn relay resistance between terminals (indicated in Fig 1 ) while connecting terminal No. 4 to the battery's positive terminal and terminal No. 1 to the battery's ground terminal. If resistance is no good, replace the security horn relay.