2003 f250

That sounds pretty normal most alarms don't arm until the dome light goes out if your not using the door triggers themselves..If you hit arm and hear the 4 beeps does it eventually arm up when the dome light does automatically go out? Also make sure you are using a positive trigger as the BLACK/ blue in the F250's shows 12v with the door open. You can also find that wire in the driver door sill about mid way down going under the driver seat. (Comes from the rear and turns down under the drivers seat, so its not in the driver kick, just FYI) Sorry don't have any info of what the actually door triggers are in that truck maybe someone else does. Good luck

Just a heads up, but most ford's need to be diode isolated because the convience module triggers the alarm after 40 minutes.  If you are using the domelight wire you may not have this problem, but if you do get the correct door trigger and your alarm goes off after about 40 min let me know and i'll send/ post the diagram.