No low temps were I am but I thought I saw little spray cans with a tube that you cans stick into the Key Slot to unfreeze locks.

Keeping the water out is another issue. You might try taping over one of the locks on the door and see if that works or not. If it does not work perhaps the Water is getting in past the Window seals.

Putting a cold metal object inside a warm garage can cause Water condensation to build up on the object. At least the Military tells you during the winter not to bring you Fierarms into a warm tent and then take them back out in the Cold. You are supposed to keep the sheltered but at the same temp as the outside thmp.

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84 300D, 82 Volvo 244Gl Diesel