Why argue with success? Especially when the road appears to point straight ahead to even greater success. That's about how Ford figured it when this kingpin carmaker (with five of the top-ten best-selling vehicles in the country) decided to badge its sport utility with the Mercury logo. Lincoln-Mercury took note of the fact that nearly 35,000 brand-loyal customers defected to buy an SUV in 1994. As a result, the Mercury Mountaineer, rolling off the assembly line this spring, was born. Everyone knows that sport utility vehicles are hot. In fact, Explorer's sales have doubled since it was introduced in 1990. It was the third best-selling vehicle in America last year and the best-selling sport utility for the past five years. But the new Mountaineer is not just a rebadged Explorer with a few accoutrements thrown in for good measure. It's a new engine, a new transfer case, changes to the front suspension and the rear axle along with a few luxury touches that Mercury thinks will fit the personality of some 25,000 to 35,000 buyers this year. With their first entry into the SUV market, Mercury hopes to attract more women and families who are looking for the standard safety features of anti-lock brakes, dual airbags and all-wheel drive in addition to a host of important attributes. After all, the Explorer's main redesign for the 1995 model year brought significant changes to appearance, performance and handling. Competitors for this upscale entry include Chrysler's Jeep Grand Cherokee, Land Rover's Discovery, Toyota's 4Runner, Nissan's Pathfinder, Mitsubishi's Montero and the new Acura SLX.