I think i found a little something ( and its really a little something ) about how to tell a 1996-1997 Sable from the 1998-1999 one from rear view. Some pics on the main site are too far or lighthing makes it difficult to see but we might be able to sort some of them out if the good conditions are reunited

1996-1997 Sable GS
http://www.rickellautosales.com/1996_Mercury_Sable_Tarpon%20Springs_FL_2297660.v- eh
LS
http://www.usedcarsgroup.com/used-1996-mercury-sable-louisville-oh-1melm53s0tg660996


1998-1999 Mercury Sable GS
http://gtcarlot.com/colors/car/19221585-9.html
LS
https://007autosales.wordpress.com/2014/10/16/1998-mercury-sable-ls-2995/

the key is the trunk's lock. Painted black for the 1996-1997 and chrome on the 1998-1999 ones, no matter the trim level.. maybe somebody already figured it out and comment about it but .... told you it was a little something...

Link to Wikipedia:
https://en.wikipedia.org/wiki/Mercury_(automobile)
Link to IMCDb listing:
http://www.imcdb.org/vehicles_make-Mercury.html



1979 Mercury Zephyr 4-door Sedan

http://en.wheelsage.org/mercury/zephyr/22199/pictures/bvc6xp/

A short timeline in order to make the topic organized:
1970's vehicles:
1979 Mercury Zephyr 4-door Sedan