Cam: I like the wording, but three matters:
1. The tag line is too small to read from much farther away than the rear bumper - it could be enlarged enough to fill the width of the sticker and that would make it far more reader-friendly. (Don't want other motorists running up our bumpers trying to read it, do we?

)
2. I think the stencil typeface is more appropriate to something "butch" or country'n'western; it doesn't - to my eye - say "German quality" or "good taste". This is, of course, an utterly subjective observation. The cleaner, clearer, less "busy" the typeface the more instantly readable it would be.
3. Whilst I appreciate the German tag line (including the possibility of its making people look twice when when they think they've spied a "dirty" word), would enough people get it to compensate for its cleverness, or would an English tag give greater utility?
Perhaps

or
