Theft Deterrent/Engine Immobiliser Systems 00' Limited Edition

Hey, I am new to this and I usually do all of my own work on my 4Runner but now I am thinking that I need to take it to the dealership because this issue is beyond me....Before I do I thought I would give this is a shot..

Here is my issue - I have a factory alarm system on it and when I lock the truck even without the remote, it's set. If I set/lock it the alarm sets off as it pleases so these days I have to leave one door unlocked so it won't go off and annoy my neighbors. I have replaced the remote batteries, the actual truck battery and neither fixed the problem. I am wondering if a sensor is going out or a fuse is blown...

Does anyone have any suggestions on where to begin with finding the cause of this issue?
Where are the sensors and/or fuses located?

 

Dutch the factory alarm they suck anyway.... Go get a nice viper 2way pager alarm .... I got 1 with remote start it's nice very long range also... Not like the factory alarm which u can barely hear when u lock it