Gotta love Excursions....
The Phoenix chip disables the PATS (Passive Anti-Theft System) that Ford thought was a good idea.
The easiest way to keep the "THEFT" light from flashing is to remove the hush panel under the dash above the pedals (if your Excursion is so-equipped) and unplug the module that is located a few inches above the accelerator pedal. It will be a 4-wire connector with either a red or green harness connector. Once that's unplugged, the light will no longer flash. Just keep in mind that if you ever remove the chip from the PCM, that module will have to be reconnected to allow the vehicle to start.