SALVADOR RIGHTIST SUFFERS A SETBACK

The coalition of five-right wing parties that dominates the Constituent Assembly here showed some signs of weakening Thursday evening when Roberto d'Aubuisson, its leader and the president of the Assembly, failed in his attempt to block a Cabinet appointment.

Mr. d'Aubuisson walked out of the morning session of the Assembly on Thursday when he saw that Dr. Napoleon Cardenas's appointment as Health Minister would be approved. Mr. Cardenas was nominated to replace a member of Mr. d'Aubuisson's extreme rightist National Republican Alliance party.

The Assembly reconvened in the afternoon, for the first time in its 10 month history without Mr. d'Aubuisson, and approved the appointment by a vote of 39 to 0.

Although it was a resounding defeat for Mr. d'Aubuisson, the moderate Christian Democrats, who were in favor of the apppointment, were cautious about interpreting the vote as a final split in the rightist coalition.

''This was a very special occasion in which we were united,'' with the other parties, said Julio Rey Prendes, the leader of the Christian Democrats in the Assembly. Mr. Prendes and seven other members of the Assembly said they received death threats between the morning and afternoon sessions of the Assembly. Government Reports Successes

SAN SALVADOR, Jan. 28 (Reuters) - Government troops have recaptured several towns and villages from guerrillas in El Salvador's northeastern Morazan Province, Defense Minister Jose Guillermo Garcia said today.

About 6,000 United States-trained troops penetrated guerrilla positions Thursday after resuming a drive that had been halted for six days, he said.

He denied reports by the guerrillas that Honduran troops were helping in the drive. The guerrilla radio said Thursday night that Honduran forces had pushed south and crossed the border in a pincer operation with Salvadoran troops advancing north to trap the insurgents.