Any luck getting this resolved? I have a 2007 Jetta and the trunk will not close when it is cold - exact same symptoms and problem as you explained; here's my details:
Today, the temperature was 42 =B0F and the latch malfunctioned. I shut the trunk and it opened immeadiately. Sometimes applying pressure or weight works and the trunk closes, not this time. Sometimes fiddling with the small, round rubber bumpers on the left and right of the latch does the trick, not this time.
So, I took my hands and rubbed them vigorously back and forth on the plastic trunk latch mechanism until it felt warm between my hands. I then closed the trunk gently and it shut...at least I could not pull it open with my hands. When I started my car, the trunk warning light was on and I could not open it via the remote of trunk button on the door. The only way to open the trunk was by folding the seats down and hitting the child-safety release. The trunk was tightly shut, but was not locked according the the computer/sensor that monitors the trunk latch.
So, with the trunk "shut," I started to drive. A few minutes later, the trunk popped open. I tried to shut it again, and again it stayed down, but the warning light was still on. This time I tied the trunk shut with a bungie-cord and went on my way. An hour later, I pulled over to check the trunk, and it was open again...I 'shut' the trunk again - by then it was 45 =B0F, still would not shut properly. I continued on. Finally, at 47 =B0F (according to my dash temperature gauge) I was able to get the trunk to properly close.
Seems like temperatures near or below 50 =B0F cause trouble for the Jetta's trunk latching mechanism.
I took my car to the dealer ( still under warranty) and told them to fix the trunk. They could not reproduce the problem the day I dropped my car off, so it looked like the trunk was perfectly fine. Turns out, it was not fine. It was near 70 =B0F that day, and the trunk works perfectly well when it is warm. SO, I am off to the dealer again, making sure it is COLD when I pull in to the dealer lot so that the mechanic can see the problem in action.