problem with 03 lincoln ls

i have  a oem ford remote start and alarm combo (code alarm) installed in a 03 lincoln ls. i'm having 1 problem with the system that no one so far can help me with. every now and then (usually when cold) when i press the start button the alarm will trigger, when the button is pressed the parking lights flash to give confimation of rear defrost turn on (although the rear defrost will only actvate after start so nothing is being activated. i changed the remote start seqence  to press lock then start and the alarm never falses. it only happens sometimes when it was progammed for a push and hold start. when the alarm falses the  system indicates the trunk pin is what set it off. i've swapped the brain to rule out defective product. i also have the same unit in a 05 aviator and never had a problem. if anyone has any idea as to what could cause this please let me know it woud be greatly appreciated.i've called code tech but they do not have an answer thanks