I had an alarm installed by the dealer after I bought the truck, got divorced and lost my garage back in 1997...
The system that they installed was called VSS-150.
The system has an override button which is silver with a small black plastic end. This is located wherever the installer thought it should be. Originally, they installed it near my left foot (lower driver's side plastic trim) which was short lived since I sheared it off while driving with my hunting boots. They then relocated it under the steering wheel.
To start the truck (and override the alarm) you must push this override in and turn the key. Truck starts immediately and the alarm goes away. However... It does not shut off the alarm and it will reactivate about 30 seconds after you turn the truck's ignition off. There should be another button which is a red light, probably on your dash. This is how you turn the alarm on and off. It flashes in a different sequence whether it is on or off. Immediately after turning the truck off, push the red button (before the 30 seconds and re-arming has occurred). You should be good to go.
FWIW, this system requires keyless entry and a fob too. Opening the door with a key will activate the alarm. The fob gives you ~ 30 seconds to start the truck before it goes off...