Stock Anti theft

[93whitestng]

I currently own a 1998 V-6 Mustang, when the temp drops below 45 degrees Fahrenheit my anti theft goes off and can't be shut off permanently unless you remove the Fuse 16 and the Horn relay. I have it at the dealer been there almost a week and they can't figure it out. The alarm system is temporarily disarmed when key is put in ignition but less than 2 mins later it is going off again. Only happens when the temp drops below 45 degrees..naturally the dealer is not able to duplicate becuase of their hours of operation. They informed me today that they(they being Ford) it is a switch in the trunk area that is telling the Anti theft system that trunk is opening and closing. Has anyone heard of anything like this, if so what was done or did you do to fix it?
Please respond to this inquiry at jaxpony@home.com thank you