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