Are you sure it is factory security? Does it ever shut off or what is setting it off? Normally if you close the doors and lock and unlock it with the factory remote it will clear it.

 

If it is factory, it is part of the vehicle security system EVS I ,II or III, if I remember right, Usually a 3" square module under the dash. May also involve the CTM which would time auto shutoff.
Pull your build sheet at any dealer parts or service dept, and go over it until you see what keyless or security system level you have.
BCM stands for body control module if that is involved I don't know.
Either whick way itis likely a dealer item ifavaialble, Maybe junk yard from like level truck. But first make sure it is factory. Any little stickers on the window?

 

Welcome to the forums, there's lots of good information here.

If you have the factory security system... VTSS... you'll have a remote fob. Close the doors and lock / unlock the truck with the fob like bigfish suggests. If you have the factory VTSS, there will be a red LED in the overhead console. My understanding is you'll have to use a working fob. Once the VTSS is triggered, if you try to start the truck the VTSS will shut it down again within a couple of seconds until the system is correctly reset.

 

Central timer module (CTM) is responsible for the security if the truck was set up like this from factory, EVS I believe that was dealer only optional installation.

 

There is 3 types of central timing mdules on these trucks the CTM is under the dash, just to the right of the gas pedal, sort of behind the ashtray, and attached to a brace.
There was a TSB for version's with problem of locking up due to strong external interference in the 315 mhz range from 2-way radios (garage door openers).

 

Yup. I've had problems with the interference issue in the past. With my factory installed VTSS, the reset I've used for this is to pull the fuse for the power doors (driver's side end of the dash... #13 if I remember correctly), wait 30 seconds, and then put it back in.

Another thought... these trucks are 20+ years old... suggest being very gentle and extra careful with all things plastic.