Mine's an '03 and there is a way to enable/disable the horn when setting the alarm (I'd have to look it up to tell you how to do it). BUT, the security light should still come on, either way.

There were a couple of mods you could do that added a switch like you mention. I have one on mine, but it's to turn on/off a rear wiper delay (thanks electroken - it's still working like a champ!). So my suggestion to you would be two things. First, as you bought the truck used and don't know what the switch is for, you could either ask the previous owner or trace it back to see where it goes. That would give you some idea of what it is for. Second, test the fob by activating a panic. If that works, you know that fob is activating the alarm. You could also lock the truck and let it sit for a short bit. Then go back an just hit the unlock button to unlock the truck. Then not do anything. Don't open any doors. After a short bit (10 minutes I think) it should relock all the doors. If that works, then I'd feel comfortable that the system is working as designed (WAD).

It may be as simple as the security light is toast/disconnected.

Report back what you find.

-G