You're able to change the FX used for the car horn, but it is defaulted to the bass guitar deploy FX. Just right click whilst in a car and you have a "horn"!
Permissions
This plugin uses the permission system. To assign a permission, use oxide.grant <user or group> <name or steam id> <permission>. To remove a permission, use oxide.revoke <user or group> <name or steam id> <permission>.