An automobile crash in which two persons were injured in Honduras earlier this month became "a happy accident" for the United States by providing new evidence that Nicaragua's Sandinista government supports leftist guerrillas in El Salvador, the Reagan administration's senior Latin America official said yesterday.

Elliott Abrams, assistant secretary of state for inter-American affairs, told reporters that the Soviet-built car had just come from Nicaragua and was headed for El Salvador when a tire burst and it crashed near Choluteca Dec. 7.

When Honduran authorities noticed wires protruding from an air-conditioning duct, they found blasting caps at the other end, Abrams said.

The wreck contained six hidden compartments stuffed with $27,400 and 450 pounds of clandestine military equipment, including 7,000 rounds of ammunition, 21 grenades, 12 radios and 86 blasting caps for bombs, as well as code books and letters addressed to the Salvadoran guerrillas, he said.

"If it is not the Sandinistas who sent the car , then it is some kind of free-lance group operating in Managua under the nose of the Sandinistas, or it is the tooth fairy," Abrams said. "Those are equally plausible alternatives."

Miriam Hooker, speaking for the Nicaraguan Embassy here, said the charges are "absolutely false." The Nicaraguan government "has denied and will continue to deny that we are providing any kind of arms or ammunition to the guerrillas in El Salvador," she said.

The administration has insisted for five years that the Sandinistas have been arming and supplying the Salvadoran rebels, but it has produced little concrete evidence.

In 1981, President Reagan secretly authorized U.S. aid to counterrevolutionaries, or contras, fighting the Sandinistas, telling Congress that it was needed to halt the flow of arms to the Salvadoran guerrillas. He is expected to seek renewed military aid to the contras early next year.

"This is a new piece of information that we think fits neatly and exactly in the pattern" of Sandinista action, Abrams said. When Nicaraguan Foreign Minister Miguel D'Escoto denied the charges in an April speech to the International Court of Justice, "he lied," Abrams said.

Abrams said proof of Nicaraguan involvement included a statement from the injured driver of the small green Lada car to Honduran authorities that he had come from Managua, had been trained in Cuba and had made a similar delivery run earlier this year.

The military equipment was padded with Managua newspapers, and the sophisticated one-time code books and encrypting material "make it very hard for the Nicaraguan government to deny complicity," Abrams said.

Papers in the car included a letter of instruction from the militant Armed Liberation Force, one of five groups in the rebel coalition, to its field commanders, Abrams said. Personal letters included one from Cuba and one from a someone in the Soviet Union written in Russian, and Abrams speculated that it was intended for a Salvadoran who had been Soviet trained.

Showing a six-minute videotape of Honduran authorities dismantling the car, Abrams alleged that it had been equipped with the compartments at a shop in Managua and said it was "a pretty good job of concealment."