I did not have a factory alarm listed on my window sticker nor did I have one added at time of purchase. As far as I knew, I did not have the alarm system from Toyota. Guess what? Most of these trucks have the alarm system in place; it is just not activated! When the alarm is listed on the window sticker or you order it, generally all they need to do is hook up the "Techstream" code reader/programmer and active the alarm option. Then they can charge you $299 for the alarm.
I found this out when I looked into and ordered the OEM glass break sensor for the OEM alarm from Sparks.
What I learned is that if your truck has an engine immobilizer (standard on V8s) and keyless entry, then you have the alarm. If it is not working it can be activated by a tech using the "Techstream". Many dealers either don't know this or what to charge you for the "alarm". Some folks have had luck getting the service guys to do it for free or for just a small labor charge. I had my Lexus tech buddy do mine in about 5 minutes.
Here is the install PDF for the glass break sensor. On pages 10-17 it explains how to activate the alarm with the scanner.
So go make friends with your local service guys and activate your alarm if it is not already active.
The guys at Sparks were very helpful in figuring this out.