Well if your C-clip looks like the one in the picture below, you can use a dental flosser and a screwdriver to push it off (and also back on). The dental flosser called fun flosser by dentek fit great ..
You can expose the clip by wedging something flat like a screwdriver between the panel and window crank (also there might be a plastic ring, between the window crank and the door panel, covering the C-clip). Hope that helps..
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