The U.S. won't impose trade barriers against Japan, the Clinton administration announced today, because of agreements reached this weekend. On Saturday, the White House reported agreements with Tokyo that will give American companies access to medical equipment and telecommunications markets in Japan, as well as the insurance and glass business. All isn't copascetic with Japan, though: the U.S. placed Tokyo on a "watch list" for its barriers to American wood and paper companies.MEXICO . . . IS THE ASSASSIN A MEMBER OF CONGRESS? The investigation into the Sept. 28 assassination of top Mexican politician Jose Francisco Ruiz Massieu, secretary general of the ruling Institutional Revolutionary Party (PRI), has found a suspect: a Mexican congressman who allegedly paid for the shooting. Jorge Rodriguez Gonzalez, arrested over the weekend, says his boss, Congressman Manuel Munoz Rocha, hired him and his brother to plan the hit. The Attorney General's office -- where the dead man's brother is deputy -- charges that Gonzalez hired the actual gumman, a 28-year-old horse trainer who's confessed. The congressman, now being sought by police, allegedly plotted the killing because Ruiz Massieu was a reformer -- just like Luis Donaldo Colosio, the PRI presidential candidate who was assassinated in March.