Nothing exotic about the Focus' electrical system. An aftermarket alarm is a stand alone system and requires very little interaction with the original electrical system of your vehicle. It just needs a power source, a connection to sense which position the key is in, a connection to sense if the doors are closed and a master to control the locks. Some systems have a sensor that detects if your foot is on the brake.

Just put it this way, Ford cannot blame your alarm if the engine blows up or the suspension has a rattle.