You have to squeeze that white plastic collar, and it should let go of the lip it's resting on. Notice that there should also be one longer tab on it; that's where you pinch it, straight in toward the cable. The cable should now slide right off the back of the speedometer. Hope this helps. I just woke up so if any of this is unclear, email me.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 