In 2004 I climbed Kehlstein and Rossfeld. I found a road to the Eagels Nest on Kehlstein where I din't see any sign that it was fordidden. I went up Rossfeld and saw a small indication for a narrow road. After I passed the forbidden road, and even after passing the "maut" station I took a narrow road on the right side. This road was not paved but only 1 km and nearly flat. Afterwards there was a wide road. Probably the forbidden one but at that point there were no signs. Soon I turned left, a quiet and narrow, good paved road. I guess the same road Etienne mentioned.
I climbed about 1200 meter in 12 km, including 2 flat km.
Most off the time you ride throug the forest (not on Rosfeld), it is not very scenery, but however, you really bike up to Hitlers Eagels Nest.
This climb is not so beautifull, not so heigh and by far not so hard as the Nebelhorn. But it surely is the nr. 2 in Germany.
Wim van Els