The mad gun man who calls himself Son of Sam claimed a sixth life today when a young woman died of a massive gunshot wound in the skull.

Stacy Moskowitz, a 20-year-old blonde, was fatally wounded Sunday while she sat in a parked car under a bright street light with Robert Violante, 20. Violante, who also was shot in the head, lost his left eye and may have lost his sight in his right eye.

While 13 shootings have been attributed to Son of Sam and his 44-caliber Bulldog revolver, seven victims have survived. Moskowitz was the sixth to die since Son of Sam's first known attack one year and three days ago.

Her parents said she died at 5:30 p.m., 39 hours after she and Violante were shot.

The 44-caliber killer's reign of terror has frustrated police, prompted hundreds of calls for action from fearful parents in all boroughs, driven young women from the streets after dark, and ruined business at discotheques, which are thought to be a favorite stalking ground of the killer.

In the wake of the latest attack, police beefed up their Son of Sam task force from 70 inspectors and officers to 300. But they soon may have another kind of violence to contend with.

Before his daughter died, teamster Jerry Moskowitz announced that several union members will meet at his house soon to form a vigilante group.

Bitterly, with fears in his eyes. Moskowitz said: "If I get my hands on him . . ."

Most of the killer's previous shooting galleries had been quiet streets, heavily shaded, out of view of nearby houses and in a relatively small area in the east Bronx and north Queens. All his victims had been attractive young women with long brown hair and the boy-friends who happened to be with them.

But at 2:35 Sunday morning the killer changed the rules of his cat-and-mouse game.

He burst from the wooded fringe of a park in Brooklyn - almost 10 miles from his previous hunting ground - into an open, well-lit street with clear lines of sight from homes a block away in each direction. Crouched police-style he fired four slugs into the front seat of young couple's car.

Deputy Inspector Timothy Dowd, head of the task force, said grimly: "We'll have to protect the entire city."

About a dozen men, including one prime suspect, were removed from police suspect lists because all were under surveillance when Moskowitz and her date, Robert Violante, were gunned down.

But police now have a victim who saw the gunman, described as white, about 5 feet 7 and 150 pounds, between 25 and 30 years old and carrying a 44-caliber revolver. One previous victim also saw him, but could not supply a good description.

A doctor at Kings County Hospital said Violante had given police that description. "He actually saw him," said Dr. Jeffrey Freedman, an opthalmologist. But because Violante lost an eye to the bullets, with the other at least partially damaged, "I don't know if he can identify him, since we don't know if he'll be able to see," Freedman said.

At one point, attendants wheeled Violante out of surgery, his shattered eyes fully bandaged. "His eyes, God, his eyes," moaned his father.

Doctors reported that Violante was out of danger, lucid and stable and talking to his parents. But surgeons had removed his left eye. They said that although the right eye could distinguish light from dark, it would probably be days before they know whether he would see again.

Meanwhile, the shadowy threat of Son of Sam is disrupting night life in areas where the killer has stuck. Bars, restuarants and coffeehouses where young people usually gather all reported a drop in business.

Shelly Shevlowe, owner of Part One, a Queens disco, said publicized expectations that the killer would strike over the weekend - the anniversary of his first murder - apparently frightened customers away.

"Friday night the place was absolutely empty," he said "It was the worst weekend I've ever has since I've owned this place."