In Hamburg, Wisniewski became involved in the left-wing scene. He protested the detention of RAF members and participated in
squattings and in the protests against the conservative
Springer press. After the death of
Holger Meins, a member of the RAF, as the result of a
hunger strike in 1974, Wisniewski joined the group. In 1975, he participated in the
West German Embassy siege. In the summer of 1976, Wisniewski was in a training camp of the
Popular Front for the Liberation of Palestine (PFLP) in Southern
Yemen. In August 1977, he participated in a bank robbery in
Essen, to finance the upcoming kidnapping of
Hanns Martin Schleyer, an employers' representative and former SS member. Wisniewski was not only part of the group which kidnapped Schleyer, he was also the one who called the shots at the scene of the kidnapping. While his collaborators shot Schleyer's driver and body guards, Wisniewski drove the van in which Schleyer was taken away. It is believed that it was Wisniewski, nicknamed
Die Furie (the fury), who later transferred Schleyer from Cologne to another group hideout in
Brussels, Belgium, in the trunk of a car. Weeks later, Schleyer was shot and killed in a forest after the first generation RAF members died in Stammheim Prison. According to
Peter-Jürgen Boock Schleyer was shot by Rolf Heißler and Stefan Wisniewski. ==Prison==