My lock was frozen too, although I was at least able to get the key in! I tried heating the key with a lighter and that worked. Afterwards, I sprayed some PB blaster in there to prevent it from freezing up again. That has seemed to really loosen things up in there for now.

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Dan
2005 E320 CDI - 246k
1987 300SDL TD05-16g, Herlevi pump, Elbe manifold, 2.47 LSD - 213k
Past: 1987 300D - 264k