F150 Anti-Theft defective.
 

Okay, my dad bought me a 99 F-150 at auction for a great price, Tonneau cover, fog lights, lift kit, aftermarket rims with low-profile wheels, modified air intake. It's great. But there are a few problems with it, one being the anti-theft goes off and wont stop once the truck starts, thank god it doesn't honk the horn (The previous owner probably disabled it) but I want to get rid of it entirely. It's not PATs, or else I wouldn't be able to start the truck. There's a check engine light on so tomorrow we're taking it in to Auto Zone or some other place and getting the ECU scanned so maybe that will get us some answers. It's really a problem because it looks like the hazard lights are on and theres an annoying clicking noise in the cabin. Any help is appreciated, thanks.

Get under the dash, find the brain box for the aftermarket alarm, and uninstall it. Restore all wiring to original.