Audio Systems: 03 Trailblazer EXT - have had vehicle starting issues periodically, trailblazer ext, allexpert

Question
Hello,

Two years or so ago, I installed a Pioneer stereo into my '03 Trailblazer EXT with the advice from a rep at the store I bought it from.  The Rep told me that I needed to install some sort of a wire harness/small plastic box that would operate my open door/warning chimes but he didn't really say anything about security.  I did purchsae and install the harness and initially did not have any problems with the vehicle not starting.  Over the last year though I have had problems where I could not get the truck to crank over.  I had it towed to a mechanic and I believe he put in a new ignition switch.  Every once in a while warning indicators light up for a while such as the airbag off indicator and seat belt indicator even with my seat belt on. Just this morning again, I turn the key and nothing happens and then turn it again and it cranks up however it seems like something is trying to keep the vehicle from running as it ran for only a few seconds.  I restarted it and it kept running.  These symptoms are the same I had a year ago where after the intermittent starting/not able to start it it finally would not start at all. Not sure but maybe I don't have the right harness or I'm not sure?  Thank you in advance for your help.  
PS - I was prompted to send you my question as I had read a response from AllExpert rep Antonio Ibarra Jr. where he asnwered someone's question on a similar topic.

Answer
The harness interface is gone bad you want to unplug the battery and pull the radio out,
plug the factory radio for a couple of weeks to reset the system and find a new harness
this is a normal problem on gm vehicles from 2000 and up that replace the factory radio,
the vehicle believe the radio was stolen because the body control module needs to see this radio every time you turn on the ignition
you need a harness like this one in the link  GM radio replacement harness interface

sorry for the early response a couple of weeks ago i was in a hurry
good luck