The debate began in 1999 with the publication of an article by Cardinal Kasper titled "On the Office of Bishop", to which then-Cardinal Ratzinger responded in 2000, The only mention made of this article is in Kasper's On the Church (America Vol. 184 No. 14 April 23, 2001. While most of the substantial texts of this debate are available online and in English, it was a source of consternation that we had not been able to find a proper citation or source for the initial essay by Kasper (disappointing many an enquirer and seminary student writing on this subject).
So I'm pleased to report the mystery is solved. In 1999, Kasper published a Festschrift honoring Bishop Josef Homeyer, a chapter on the theology and praxis of the bishop with special reference to the relation of the local bishop to the universal Church.
The article alas, remains unavailable online. But for an analysis of its key points, see Walter Kasper on the theology and the praxis of the bishop's office, by Kilian McDonnell. Theological Studies Vol. 63, 2002.
The full citation for Kasper's article is "Zur Theologie und Praxis des bischöflichen Amtes," in Auf neue Art Kirche Sein: Wirklichkeiten-Herausfoderungen-Wandlungen, ed. Werner Schreer and Georg Steins (Munich: Bernward bei Don Bosco, 1999) 32-48, at 43.].