'99 Ranger 4x4, 5-speed, bought it used 5 years ago with one key and no security fob. The truck ran great until last fall. The check engine light came on, so I disconnected the battery to reset the computer. The truck started and drove fine. Later I went to start it, it would not start and the alarm (horn relay) went off. I am using the only factory key. Disconnected the horn to make it easier to work on. Battery has a full charge. After my son and I played with different combinations of locking/unlocking the doors and inserting the key, the engine started. I drove it for a day, then it wouldn't start and the horn relay went off again. I checked the manual, this truck does have passive anti-theft. I took out the battery and left it in the garage for the winter. I want to get this nice little truck running again. Any ideas before I have it towed to my mechanic?

 

If engine started at anytime then PATS is not the problem, it will be the Factory alarm system, the two are not connected in any way, and PATS doesn't use the horn

So your key is not the problem but I would get it "cloned" at a locksmiths shop sooner than later because if Key does fail it can get expensive for towing or mobile locksmith to come out.

For the alarm
Remove drivers side kick panel
There will be a Dark Green/purple stripe wire usually running into the doors wiring harness
Use a wire splicer like you use on trailer wiring to splice in a wire to the Dark Green wire
Now you need to GROUND the spliced wire to a body ground which should also be in the kick panel area.

Grounding the Dark Green/purple stripe wire is the same as opening drivers door with the key(or key fob), it turns OFF the alarm when this wire is grounded, ungrounded the alarm is on.

If this doesn't disarm the system then problem is most likely in the Alarm "brain" itself, which you will have to locate

One common failure point on Factory alarms is the HOOD OPEN plunger, open the hood and have a look for the plunger, unplug its wire connector, see if alarm works as normal now.
It may use same plunger as hood light or have its own, so have a good look