Does the cylinder rotate at all? If it does and it's sticking moving in the opposite direction, it's more than likely the problem is with the central locking linkage or actuating cylinder, but just in case it isn't,try and lube the lock cylinder with either WD40 or graphite. I prefer the WD40 b/c it comes with a neat little spray tube that you can gently push into the cylinder. I don't know what you're weather has been like, but if it's been cold you may have some h2o in there. The WD40 will displace the moisture and will not make a gummy mess like the graphite. Good luck

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Jeff L
1987 300e
1989 300e