From $600 rebates to the new no-frills Taurus and Sable, Ford is finally getting the message about affordability. The bare-bones midsize sedans, both available in April, will come without cruise control, floor mats, power door locks or a cassette player. All interiors will be gray. You can have any exterior color you want as long as it's white, red, gray, silver or green. The Taurus G will be stickered at $18,545 and the Sable G at $18,91 0, both including delivery and destination charges. Meanwhile, to catch up with Chrysler, which introduced its current generation of minivans with sliding doors on both sides, Ford this fall will introduce a "superdoor" on the driver's side of its Windstar minivan. Retooling for the change will cost an estimated $115 million. Combined with a driver's seat that flips forward, the larger door will allow children to slip through to the middle bench behind the driver's seat. General Motors Corp. will have two sliding doors on its new minivans coming later this fall. Ford is investing an additional $500 million to give Windstar a driver's side sliding door by the 1999 model year. Chrysler officials say about 85% of their minivan customers order the $500 option.