IT'S TIME to retire the Plymouth Reliant K car. This compact econobox has done its job. It saved Chrysler Corp. from ruin. It launched a whole new generation of front-wheel-drive vehicles. It proved that America can make cars that make sense.

But the K car always had shortcomings, particularly in styling.

The car is a hymn to automotive asceticism. Its exterior lines are austere, severe, depressing -- the essence of the Sackcloth School of Design.

That kind of stylistic humility might have been appropriate in 1979, when the first Reliant K was introduced -- and when a contrite Chrysler was begging the government for loan guarantees and a second chance on life.

But the car is hopelessly out of date today. A look at Chrysler's full line of current models shows that much. Throw in foreign competition -- spiffy, classy, fuel-efficient cars from Germany, Japan and Korea -- and the Reliant K looks and feels even worse.

It's time for Chrysler to face up to it. The Reliant K is past its prime. Let it go in peace.

Outstanding complaints: The 1986 test-model Reliant K is equipped with vinyl-covered front bench seats; and that's an outrage, even in an economy package.

The human body sweats through clothing and sticks to vinyl seats, even with the air conditioner running. And when the body isn't sticking, it's sliding from side to side, because the vinyl benches lack lateral supports.

But what really does the body in are the stationary backrests on the test car's benches. There is no reclining, nor sitting up a little straighter, in this one. Your body either fits or it doesn't.

Chrysler offers cloth reclining front bucket seats as options. Spend the extra money.

And this: Chrysler's assembly plants have been running full blast, and it's a wonder that the company has been able to maintain a generally high level of product quality. But there were several fit-and-finish blemishes in the test model that indicate that some Chrysler workers oughtta take a break. Chief among the goofs were ill-fitting interior decorative door panels, and some sloppy welding and putty work where the doors are joined to the car body.

Outstanding praise: If utilitarianism were a virtue, the Reliant K would be a saint. It has adequate space for six adults and their luggage, especially if they are traveling with crushable bags. You can park the Reliant K almost anywhere. Most auto thieves will ignore it.

Ride, acceleration, handling: The test model rolls over most roads rather nicely, which is a blessing, considering the occupant punishment inflicted by the seats. The car is equipped with an optional 2.5-liter, 4-cylinder engine, which provides substantially better acceleration than the workaday 2.2-liter powerplant that is standard equipment in the test model. Handling is decent at legal speeds,

Sound system: AM/FM stereo by Chrysler. Good.

Mileage: About 24 to the gallon (14-gallon fuel-tank capacity), combined city-highway, carrying five occupants and running with air conditioner on.

Price-as-tested: $10,487, including $2,280 in options such as the 2.5-liter engine and three-speed automatic transaxle.