Have done these in various stages of desperation/frustration:
1. Lit matches on key (nope).
2. Lit propane torch on key held by vise grips (yep).
3. Hair dryer (nope).
4. Electric heat gun (yep, but the paint!).
5. A shot of WD-40 in the fall before freezing weather (yep).
There are little aerosol cans of lock de-icer at most stores, but it seems to me that it wouldn't work too well laying in the glovebox of an iced vehicle. Some say WD-40 isn't good for the locks but it works for me and I've never had a lock failure. They also get a shot immediately after washing and before leaving the heated building.

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daBenz - 1970 220D