Here's a link to the Nürburgring 24h-site:
http://adac.24h-rennen.de/
Check out their "Archiv" under 1982-1992 for the 1984 edition of the race. Niedzwiedz/Doering/Gartmann retired their Ringshausen-Mustang - there's even a picture of the car. I guess the drivers must be Klaus Niedzwiedz/Helmut Döring/Dieter Gartmann.
Jesper