I can turn the passenger side knob to warm, but the drivers side is stuck. How simple of a job is it to remove/lube that cable? Or could it be something frozen at the other end of the cable?

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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D

Get under the hood and take some pliers to the cable. The passenger cable was frozen on mine but I was able to unfreeze it from under the hood. Then sprayed some lube on it and it is as smooth as the driver side now.