If you want a real American six-passenger sedan that has rear-wheel drive, body-on-frame construction, lots of luxury amenities, and a reasonable price, you are shopping in a very narrow segment. Chrysler Corporation gave up on rear-wheel-drive passenger cars years ago, and 1996 was the last year in which General Motors made any large sedans with rear-wheel drive (the Cadillac Brougham and the Chevrolet Caprice were canceled after that). That leaves the Ford Crown Victoria for the police forces, daily rental and taxicab companies, and the Mercury Grand Marquis for the rest of us. The Grand Marquis has posted strong sales gains over the past few years. Mercury officials credit changing American demographics - more older couples who have children or parents living with them, for instance - and the car's solid value, an important factor for those on fixed incomes. Only four years ago, the Grand Marquis had less than 15 percent of the large-car market, now it's closer to 27 percent. Owner loyalty, which speaks volumes, is more than 42 percent, the highest in this part of the market. Grand Marquis competes against Oldsmobile Delta 88, Pontiac Bonneville and Buick LeSabre as well as Chrysler Concorde and Toyota Avalon.