Ford Motor Co. is taking plenty of heat these days for failing to offer a fourth, rear passenger sliding door on its Windstar minivan, which debuted in 1994. Meantime, General Motors Corp. engineered a fourth door on its all-new '97 U-van see cover story, p.26), and Chrysler Corp. offers it as a $500 option on its new-for-'96 minivans. As a stopgap before the next Windstar comes along -- likely 1999 -- Ford's tearing up the vehicle to offer a driver's side door that's about a half-foot wider so rear passengers can slip in and out. A weary Ford insider says the "superdoor" fix is plain stupid. "It's going to be heavy, and what's going to happen when it swings out in a close parking situation?" A permanent fix will be even more costly: Windstar's fuel-filler sits where the fourth door would go.