As with the Cheshire cat, the only thing remaining from the 1988 Mercury Cougar is its smile. That is, the emblem on the grille of the '89 model is unmistakably Cougar, but everything else has disappeared.

No halfway measures or facelifts here but an entirely new design. And although as in the past it shares much with the Ford Thunderbird, it comes through with its own look and personality.

Just to see how complete the change has been to this rear-drive mid-size specialty car, all one has to do is take a look: it is lower, wider and shorter but it has a much longer wheelbase. The last part may sound like somewhat of a contradiction but that's the way it works. The wheelbase is nineinches longer and the car is two inches shorter. It is also an inch wider and lower.

Together with new sheet metal that is aerodynamic, though not overly so, the new Cougar looks sleeker, more aggressive and richer, the latter certainlynever a handicap in this particular market segment. In fact, it is beginning to look more Lincoln than Ford.

Although it is not quite as apparent as the styling changes, the basic platform incorporates a new chassis that includes separate front and rear subframes, a new short- and long-arm front suspension and a new independent rear suspension system. You really don't have to know what this means to enjoythe Cougar but it is all rather sophisticated and quite an advance.

The Cougar comes in two series, the LS (lots of luxury) and the XR7 (lots of sport). The test car was an LS and would be suitable for the individual whoenjoys luxury in a coupe and/or, because of its generous size, for the young family.

Just to cover some basics, the new Cougar has a wheelbase of 113 inches, length of 198.7 inches, width of 72.7 inches, height of 52.7 inches and curb weight of 3,553 pounds. It has an EPA rating of 116.9 cubic feet (102.2 passenger and 14.7 cargo), making it a mid-size (110-119 cubic feet) that is close to being a large car (120 and up).

The interior is roomy and can easily accommodate the five passengers it is rated for. Leg room is exceptionally good for a coupe with a maximum of 42.5 inches up front and a minimum of 36.7 in the rear. Trunk room, at almost 15 cubic feet, is decent for a mid-size car though not outstanding. The trunk lidhas a low lip and loading and unloading are very convenient.

As with many other cars these days, the Cougar has the passive shoulder restraint, which at best has received mixed reviews since its introduction several years ago. In addition to startling first-time passengers to this system, the passive belts (on all cars, not just the Cougar) prove to be such an annoyance to many drivers that they are disconnected - an obviously counterproductive move. Since the passive shoulder restrains still require front-seat occupants to manually buckle up the lap belt, there really doesn't appear to be any big gain, especially since so many state s have passed seat- belt use laws.

Anyway, the Cougar's interior is roomy, comfortable and loaded with creature comforts.

Driving the Cougar isn't all that difficult. A large glass area with only aslight blind spot on the sail panel (most cars have some sort of blind spot) provides acceptable visibility. Providing somewhat of a sporty air is the full-length console with floor shift for the four-speed automatic overdrive transmission. The console also has an armrest with storage box and several smaller storage trays. The interior also has a redesigned electronic instrument cluster with fuel-distance computer and tachometer.

One of the options on the test car was ABS (anti-lock braking system), one of the great, if not the greatest, safety innovations of the decade. The outstanding feature about ABS is that it helps a driver maintain steering control even during hard braking on rain- or ice-slicked pavement. And on slick surfaces ABS can reduce stopping distances y up to 25 percent. (Keep inmind, though, there is no braking system that can stop a car as fast on ice ason a dry surface. A car will still slide with ABS but it will be a straight, controllable slide.)

ABS accomplishes its task by not allowing the wheels to lock up. Sensors monitor the rotation of each wheel during braking and feed this information toa microprocessor that controls brake pressure to each individual wheel. If anywheel is about to lock up, the pressure to that brake is modulated to provide maximum braking efficiency without lock-up. In effect, the system pumps the brakes up to 10 times per second - much faster than even the best of drivers can do.

As mentioned, the Cougar's four-wheel independent suspension is rather sophisticated. This is the first year that the Cougar has an independent rear suspension, a configuration common to front-wheel drive cars but not yet across the board on rear-drive cars. Even though the LS's suspension is tuned for the luxury ride, handling is still quite good. And more so with the optional GT-Plus-4 tires on the test car.

The Cougar is powered by a 3.8-liter/231-cubic-inch V-6. This engine has been used extensively in Ford Motor Co. cars for several years now and just keeps improving with age through refinements. The version in the LS is the standard 3.8, featuring multi-port electronic fuel injection and rated at 140 horsepower at 3,800 rpm and 215 foot pounds torque at 2,400 rpm. Performance is lively for all Lehigh Valley driving conditions but far from breathtaking.

The XR7 is the breathtaking performer. It is powered by a supercharged - note, supercharged, not turbocharged - version of the 3.8 V-6 rated at 210 horsepower at 4,000 rpm and 315 foot pounds torque at 2,600 rpm.

Fuel mileage for the LS was very decent for a 3,500 pound, mid-size car. The test car averaged 14 miles per gallon for city driving and 23 mpg over thehighway. And as a bonus, all on unleaded regular.

Full price on the test car came to $19,171, not exactly a cheap figure but just about what one can expect to pay for a heavily equipped mid-size specialty coupe. Base price is $15,448 and includes a long list of standard features such as air conditioning, AM-FM stereo with four speakers and digitalclock and power windows.

Options included: Preferred Equipment Package 263 (tilt steering, speed control, rear window defroster, P215/70R15 BSW tires, cast aluminum wheels, power lock group, high-level AM/FM stereo with cassette, leather wrapped steering wheel, dual 6-way power seats, power antenna, keyless entry system, diagnostic maintenance monitor, luxury light group and luxury lamp group), $2,695; performance ''Traction-Lok'' axle, $100; anti-lock brake system, $985,and JBL sound system, $488. Delivery is $455. The price reflects special addeddiscounts of $1,000.

The Cougar is protected by a six-year/60,000-mile powertrain warranty and asix-year/100,000-mile cor rosion perforation warranty.