Car Alarms: 96 Chevy Blazer alarm system, chevy blazer, arm function

Question
I bought a used 96 Chevy Blazer.  It has, what appears to be, a factory alarm that was installed after the vehicle was built.  The unit under the dash has GM stickers and TRW molded into it.

The problem I'm having is that it auto-arms itself after 15 seconds of sitting.  I'd like to utilize the alarm, but I'd rather it didn't auto-arm itself at all.  

I've tried the reset solution I've read about on this site, but it doesn't seem to work for me.  It has the valet button under the dash, which I've pushed while the key is in the "on" position.  It didn't seem to do anything.

Any thoughts on how I can cancel the auto-arm function?

Thanks in advance!

Answer
No, not factory.
  Factory alarms DO NOT passively arm themselves.
So yes, it does have an after market system.
That button you found may be damaged or, has another
procedure for valet/overide, but to answer
your auto arming question, unless you can
ID the system, you won't know how to re program it
to turn off the auto arming feature.

Sorry, that's what I know.

ALARMTEK