Took a look at a 99.5 A4, the brace is bolted between the aluminum "plates", at the bottom of the rear engine cradle-to-chassis mounts. On a S4, it is welded between the ends of the engine cradle itself. This difference seems odd, as the A4 brace would stiffen the unibody at the cradle mounts, while the S4 brace stiffens the engine cradle itself, not the unibody. I wonder about the effectivness of the A4 brace, as the aluminum "plates" it bolts to don't seem like an overly ridgid interface to the unibody.