I'm not positive, but this sounds like one of those alarms the dealers install and then try to sell it to you for an extra 4 or 5 hundred bucks. If you don't buy it they leave it in the vehicle and don't arm it. Problem is, you are looking at a red light for as long as you have the truck or you can remove it and have a nice hole in the dash.
I passed up a deal on a new car about 5 years ago because of this "sales tactic". We had a deal made until they told me they would not replace the dash piece they had drilled for the light because "it was an expensive part". I told them they should sell the car before drilling holes in it that the new owner might not want and walked out of the deal.
BTW - this alarm system they offer is the same as one of those $59 cheapy systems you can buy at any discount parts store.
If what you have in your truck is what I have described, your options are to replace the dash piece they destroyed for you or cover the light with tape or something.
Sorry - Good Luck