The Grand Marquis is almost the same car as the Ford Crown Victoria, but it has more luxury options. That makes it more suited for the consumer market than the Crown Vic, which is very popular with taxi and police car fleets, two groups that do not need luxury options.
The Grand Marquis is sold with 5 trime levels: GS, GS Convenience (yes, that's the actual name of the trim line), LS Premium, LS Ultimate, and LSE.
The first generation Grand Marquis was sold from 1983 to 1991. It was a boxy car based on the Panther platform. It had a Windsor V8 engine and a four speed automatic.
The second generation models were sold from 1992 to 1997. It had a Modular V8 and could get 20 miles per gallon.
The present generation started in 1998, and it's fair to say that at this point it's out of date. It still has the Modular V8 engine, and it offers very little over the competition from Cadillac and other luxury car makers. The Palm Beach package is one plus. It includes leather interior and special badging.
This car will be made at least through 2010, according to Ford, the parent company of Mercury.
Grand Marquis' are easy to find on the used car market. If you look on eBay right now, you'll find plenty selling for a fraction of their new price, and lots of parts for fixing them up as well.