Hector Ratto was 29 at the time and a union activist at the Mercedes Benz plant in Buenos Aires.
Ratto, testifying on Tuesday at the trial involving the last junta leader, retired general Reynaldo Bignone, described his 19-month detention and torture at an army base in explicit detail.
He said that soldiers kidnapped him from his boss' office at the car manufacturer's plant in August 1977, and that he overheard his boss - identified as Juan Tasselkraut - giving by telephone the home address of another activist who was arrested and is now presumed dead.
"It is evident that the addresses came out of the company," Ratto said. Those arrested were union activists but otherwise not interested in politics, he added.
Ratto was released in March 1979 and told not return to his old job.
The company had paid his wife part of his salary during the time he was detained, but contributions to his retirement benefits had ended.
Julio D'Alessandro, who worked at the factory at the time, told AFP that the army's ties with Mercedes Benz predated the dictatorship, as they were big buyers of Mercedes Benz Unimog trucks.