The son of an industrial clerk and a seamstress, Rettner completed an apprenticeship as a bricklayer in Zeitz and Gera after attending primary and secondary school, and then worked in that profession for some time.

[1] In 1989, he briefly also served as head of the clandestine Central Committee Trafficking Department, responsible for courier services and secret financing of the West German Communist Party, succeeding Julius Cebulla.

As head of the Department for International Politics and Economics, he had significant contacts with West German, SEW (Socialist Unity Party of West Berlin)[5] and leading SPD (Social Democratic Party of Germany) politicians such as Jusos chairman Gerhard Schröder, Anke Fuchs, Peter Glotz and particularly Minister-President of the Saarland Oskar Lafontaine, including from the Berlin state association.

[7] As the head of the Central Committee department responsible for Western policy, he wrote a report to SED General Secretary Erich Honecker in March 1987 about a meeting with Harry Ristock, a longstanding figure in the Berlin SPD.

"[8] In the fall of 1987, a meeting took place with another senior SPD politician, Karsten Voigt, a member of the Bundestag and the party's foreign policy spokesman.

He rejected any "interference in internal affairs," described the detainees as "people who acted against the laws of the GDR," and advocated "a realistic approach to normalizing relations between the two German states."

In the protocol that Rettner prepared for the Politburo afterward, it was noted that Lafontaine "was visibly affected" and responded that it had "never been his intention" to "discredit Erich Honecker's policies," and that he had made his statement "primarily from an internal political perspective."

[1] On 1 October 1989, he received a letter from Dietmar Ahrens, then SEW Chairman, reporting on a conversation between Berlin's Governing Mayor Walter Momper and Valentin Falin, Head of the International Department of the CPSU Central Committee.

In a four-hour conversation on 15 November 1989, at the East Berlin Palace Hotel, Rettner and Kiep prepared a visit from Chancellery Chief Seiters.

[14][15] In 1990, Rettner, alongside Hartmut König, another former Secretary of the Central Council of the FDJ, worked on Egon Krenz's book "When Walls Fall.