Security light on, horn keeps going off.

Try checking the horn relay, pull out the wires if the horn stops then the button is broken, it not then it is the relay. With your security light on your antitheft system is failing. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM. These 3 parts are in CONSTANT communication. If the key is in the ignition, these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. There is where the security light turns on and shuts down your vehicle.