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Trunk door hard/impossible to open in cold weather

I have a 2003 Mercedes Benz C320 station wagon. We live in Florida and spend part of the summer in the Rockies in Colorado. Beginning last summer, when the temperature fell to the low 40s the cargo door became hard to open and when the temp fell to the low 30s it would not open at all. My Florida mechanic thought the problem was moisture in the locking mechanism & sprayed it with WD 40. But that does not explain why the door is hard to open in the low 40s. Could it be something else? I would like to fix it before we go back to Colorado. Thanks!

Are you saying the latch itself won’t work in the cold, or that the door is harder to move up and down? The struts that hold the door open get stiff in cold weather, and the door will take more effort to lift. That’s normal. Why the latch wouldn’t work in the cold is a mystery to me.

Are you trying to open the door with a key or with an electronic remote? Is there a separate exterior handle for the cargo door?

My Florida mechanic thought the problem was moisture in the locking mechanism & sprayed it with WD 40.

The problem now may be the WD-40. WD-40 is not a lubricant. It is a water displacement. So while it might have fixed the problem if it were water in the lock, it could have made other problems worse, or maybe temporarily better.

WD-40 is a witches brew of various oils and stuff. It makes a good solvent for some things and a good lubricant for a few, but when it has been used in a lock, it is wise to follow up with a good lock type solvent - lube. I suggest Lock-Ease. It has some non-residue solvents and graphite dust. The graphite will lubricate the lock without attracting other dust which will be bad and will not get gummy.

I use the electronic remote to unlock the system. Then I squeeze an exterior handle to release the latch and I manually lift the door. When the temperature is in the low 40s and I squeeze the handle, the latch releases but it’s very hard to move the door up. When the temperature in the low 30s and I squeeze the handle, I am not sure that the latch releases; the door just will not move. A friend who used to work in a body shop managed to get the latch to release - and then lift the door up - by banging with his fist on the panel below the latch. As a test, he closed the door and it took a while and a lot of banging to get it open again. The latch does not seem to release AND the struts seem to resist.

Thanks! I will get Lock-Ease and try it.