NARVACAN, Philippines (AP) _ Opposition leader Corazon Aquino took her presidential campaign into the home country of President Ferdinand E. Marcos for the first time today, attracting 3,000 cheering listeners in an area Marcos claims is solidly behind him.

Mrs. Aquino and her running mate, Salvador H. Laurel, rode atop a small truck into this small coastal town in Ilocos Sur province north of Manila as hundreds of people lined the way, throwing confetti.

They waved signs saying, ''No More Solid North'' in reference to Marcos' expectations that the area will solidly support him in the Feb. 7 special election because he is from neighboring Ilocos Norte province.

In other developments, Cardinal Jaime L. Sin and six other Roman Catholic bishops in Manila urged the government Commission on Election to count votes ''honestly.''

In a pastoral letter read Wednesday at churches, the clergymen said, ''It is seriously immoral and un-Christian to cheat or make others cheat during the elections.''

Also Wednesday, Armed Forces Chief Fabian C. Ver issued a new code of conduct for soldiers in an apparent effort to improve the military's image before the election. The guidelines include admonitions to ''respect elders and local officials. Respect females.... Pay for food and drinks you take.''

Ver and 25 other men were acquitted last month of charges in connection with the 1983 assassination of Mrs. Aquino's husband, Benigno, a Marcos foe.

Speaking in Narvacan to about 3,000 people, Mrs. Aquino decried monopolies in tobacco, the local crop, and pledged to stop garlic smuggling from Taiwan, which she said has hurt the region.

Vigan, the capital of Ilocos Sur, was Mrs. Aquino's first stop in a planned two-day tour of the northern region. She visited a market and spoke briefly to about 200 people.

Mrs. Aquino was to have opened her Ilocos campaign in Marcos' province, but canceled the stop at the last minute because of what aides called ''time constraints.''

Ilocos Sur was the 20th of the Philippines' 72 provinces that she has visited during 22 days of campaigning. Marcos has limited his campaigning to Ilocos Norte and areas close to Manila.

Ramon Incarnacion, opposition provincial chairman for Ilocos Sur, said Marcos' party, the New Society Movement, won both National Assembly seats from Ilocos Sur in the 1984 election with 70 percent of the votes.

But Pablo Sanidad, regional opposition leader, told the Narvacan crowd that Marcos has lost many supporters in the region. ''The solid north of Marcos is dissolved,'' he said.

Narvacan Vice Mayor Sally Villanueva said she had quit Marcos' New Society Movement three days ago to support Mrs. Aquino.

''I am an Ilocano but I am a Filipino first. I am tired of all the lies. We cannot recover with President Marcos, it's useless,'' Mrs. Villanueva told The Associated Press.

However, other people interviewed in Narvacan and Vigan said they still would support Marcos. A shoe salesman who refused to give his name said he would support the president ''because he is an Ilocano and because he has given us so much aid.''

Incarnacion said the province accounts for 269,000 of the nation's 27 million voters.

In Vigan, two dozen mostly foreign journalists crowded around Mrs. Aquino as she shook hands with surprised vendors in the dark, one-story market.