Car Alarms: Ford Explorer 02 Factory alarm, purple wire, office position

Question
I have a 2002 ford explorer xlt.  It seems latley that the factory alarm goes off when I open the door.  I have another installed alarm with auto start for about a year and never had a problem with it.  Also I have noticed if I keep the dimmer switch locked in the office position the alarm does not go off.  Please advise what might be the problem.  

thanks

Answer
Keeping the dimmer switch in that position keeps the factory alarm from arming.  The aftermarket system you have installed should offer a factory alarm disarm output, that pulses a green/purple wire with a ground.  This wire is also the wire that gets a ground pulse when you unlock the driver's door with a key.  It sounds to me as if this wire was not hooked into when the aftermarket system was installed.  If you post the exact make and model of your alarm I may be able to locate the manual for it and let you know which wire needs to be hooked to that green/purple.