Each year, fewer and fewer customers line up for traditional full-size American cars. These rear wheel drive behemoths are being replaced in the public's affections by a new breed of smaller, lighter, more fuelÐefficient cars. Most people think these time-honored all-American sedans are on their way out. Chrysler and GM certainly do. It's been years since the former has built any body-on-frame passenger cars, and this year will mark the end of production for the Chevrolet Caprice, Buick Roadmaster and Cadillac Fleetwood, the General's final entries in this class. Which leaves Ford Crown Victoria and the Mercury Grand Marquis as sole occupants of a dwindling market. However, there are people who still appreciate cars like these. Taxi fleets and government agencies, for example. Body-on-frame construction, which entails separate sub-construction for the chassis and body with the two mated in final assembly, is generally heavier and better suited to taking a beating than the near-universal unitbody approach. That's why most trucks and sport-utilities are still body-on-frame designs. Custom coachbuilders like separate frames because they can stretch them easily. Police forces like the high-mileage reliability of rear wheel drive powertrains, a roomy back seat for passengers likely to be wearing handcuffs and a big trunk for emergency gear. And large families still enjoy filling that large trunk with luggage and setting out, six at a time, on long trips. Ford figures that's a big enough market to warrant continued production of Marquis and Vic, and to make continual improvements to them. Given the appeal of the current cars and the ever-dwindling number of competitors, they're probably right.