I have a 1998 For Ranger XLT with 55,000 miles. When I shut the ignition off, open the door and lock the door with the power door lock, the anit theft light comes on and starts blinking rapidly and I cannot get it to shut off. Not sure if it is worth mentioning but when I pushed the power door lock down to lock, the horn beeped once and now with the light blinking, the horn will honk each time I hit the unlock or lock button. I had the light come on once before another time but was able to get it to shut off by either unlocking the door with the key, using the power door locks or just turning off the iginition and back on again. Nothing is working now to make it stop blinking so I have disconnected the battery to keep it from running down. It does not affect the truck in regards to starting.

 

From my 2000 Owners Manual (you can get one for your year as a pdf copy from Ford's web site):

"The theft indicator on the instrument cluster will operate as follows:
* When the ignition is OFF, the theft indicator will flash briefly every
two seconds to indicate the SecuriLocky system is protecting your
vehicle.

* When the ignition is turned to RUN or START, the theft indicator will
light for three seconds and then go out. If the theft indicator stays on
for an extended period of time or flashes rapidly, have the system
serviced by your dealership or a qualified technician.

The Theft indicator is controlled by the SecuriLocky system. During
the 30 second prearm state, it is normal to see the theft indicator pulse
brighter behind the constant glow."

From your description, the theft light flashing in your instrament cluster is working correctly.

 

Good news is it is not a Ford PATS system, Rangers didn't get that until '99 model year.

If horn sounds and not a siren then it is a Ford Factory alarm system not an add-on or Dealer system.

Common issue with these are the door lock/unlock sensors, since you had a problem with the drivers door before I would check it again, and the passenger door, usually spray lube can fix the problem and doing the lock/unlock several times.
Do you have a "door open" light?
Make sure it is working, i.e. start truck and open drivers door to see if it comes on, and do the same for passenger door, I believe that uses the same sensor as the alarm.
It "door open" is not working then alarm can't set.

Next is the ignition key tamper switch, stay inside and lock truck, wiggle key switch and see if light flashing changes.

There is also a hood switch sensor, a little push button switch, usually on passenger side, it is pushed closed when hood is closed so alarm can activate, if it is not closing then alarm can't set and will flash a warning.

Check fuses #18 and #24

 

Tried the spray on the door latches and this did not do any good, light blinks very, very fast. Door ajar light and alarm comes on when either door is open and light continues to blink rapidly with doors closed and engine running.

 

Took the truck to Ford dealer and will just hope I don't have to take out a 2nd mortgage after it is fixed.

 

I guess all things are related because my key fob not working and then problems with my anti theft light flashing rapidly ended up being a broken wire and a bad connector near the RAP module. Total cost from FORD dealer was $363 for repairs but my anti theft is working and not draining the battery or going off and my key fobs open and lock the doors. I was told the connector was "obsolete" and they had to repair it another way.

 

Thanks for the update