Alcohol and WD-40 have both worked for me. They temporarily purge or displace the moisture in the lock tumbler that is freezing up. Actually, the WD-40's effects lasted and made the lock action really really smooth.

Alex

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1983 300D (parked for four years)
2012 VW Sportwagen TDI Manual
2001 Miata SE
1962 Chevrolet Corvair Rampside