GO TO
Squarebirds- 1958 1959 1960 Thunderbird Shift Lever Detent Plate And reference to your problem see also.. there is from the august #95 issue of CLASSIC CAR MAGAZINE available at 1-800-227-4373 (page 7 8 ) see shiftless squarebird story.
there's a good tech. article detailing these shifters that have popped out of park and feeling sloppy in other positions! don't loose the two # 4 machine threaded 1/2 inch screws that hold your signal assembly flat in place in your steering columb. invest in a magnetic screwdriver for the easy fix desribed in article.