Dan,
This looks very similar to the speedo cable end on my Capri. If Hershal's suggestion doesn't do the trick, you may want to try this - for my cable I had to deflect the lower (closer to the speedo) ring on the plastic connector until it contacted the speedo casing while keeping the cable straight or with slight pressure on the cable itself in the opposite direction while sliding it out. By the looks of your connector I would suggest pushing the side of the lower ring with the single leg towards the speedo housing (see photo). If this doesn't work try the opposite direction. This is real bear especially when working under the dash.
Regards,
Al