whatever you do... DON'T FORCE IT!!!!! I am a certified locksmith, and forcing it is not the best way to go, if you do, you'll break the key off, thus incurring a broken key extraction. try some wd40, but what probably happened is either your key or wafers have worn down, or the lock is gummed up with old grease, as was the case with my sd's doorlocks. i had to remove the lock and "de gum" it.

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion