Need Help With a Power Drain

I'm a 5.0 guy, but my buddy has an 01 GT with an 03 Mach 1 engine in it. Every time he drives the car and turns it off, some sort of anti theft thing somewhere in the car starts clicking constantly, never quits, and in turn keeps killing batteries if he doesn't drive the car enough. The car does not have an aftermarket alarm system so whatever this is has to be factory. The car is a bit of a Frankenstein since it was wrecked at some point and it was rebuilt and a Mach motor with forged internals was thrown in. He doesn't car if the factory anti theft works or not, so is there some way the system can be disabled etc. so we can eliminate this problem. Any ideas would be greatly appreciated.

I'll check into that, but i think it's something with the anti theft system. the light on the dash flashed constantly and there is a distinct clicking sound under the dash/hood that we cannot quite locate.