To remove the chrome shifter is actually pretty easy once you understand the parts they used.
The chrome shifter has some large rubber washers and two different snap collars. The one that has you stymied is the one at the very bottom. It is a crenelated plastic part with a ring, the top of the crenelations have an outwardly radiused protrusion. The radiused protrusions interact with the bottom of the shift lever. Releasing this part is the key to removing the shifter.
See this illustration to understand the parts involved, the offending part is #1 in this illustration:
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If you put the car in reverse or in fifth you will have a better chance of seeing some of that part peaking out of the bottom of the shifter. Using a small straight blade screwdriver or preferably a pointed pick with a 90° bend, reach between two of the crenelations and lever the plastic part down and out of the bottom of the shifter. You may need to use two tools in combination to get a few of the sections of the part to disengage. Once the clip is loose, shifter can be pulled off which may be very easy if the rubber parts are worn or surprisingly difficult if they are newish.
A word of warning, the part is plastic, it is old and as a result could well break. Happily it is available from all of the various vendors as part of a kit so you will be able to reinstall the shifter. If you don't replace the bottom part the shifter will periodically pull off depending on your shifting style.
http://www.fiatplus.com/shift-lever-bus ... 84613.html
http://www.vickauto.com/cgi-bin/store/76-1300.html