7PM This episode's headline: Donald Regan: On the Spot; Nixon Resignation -- Ten Years Later; Aid for El Salvador -- Arming the Rebels; Tales from Route 89: Arizona; Dwight Stones: Golden Oldie. The guests include In Washington: DONALD REGAN, Treasury Secretary; THOMAS PICKERING, U.S. Ambassador to El Salvador; Rep. STEPHEN SOLARZ, Democrat. New York. Byline: In New York: ROBERT MacNEIL, Executive Editor; In Washington: JIM LEHRER, Associate Editor; JUDY WOODRUFF, Correspondent; Reports from NewsHour Correspondents: JIM FISHER (Kansas City Times), in Abilene, Kansas; PHILIP HAYTON (BBC), in Ciligny, Switzerland; LARRY MERCHANT, in Los Angeles