1993 BMW 325

First of all, it's not in the exhaust manifold on a 93 as it is in the 96 through 98 versions of the same car. It's in the section of the exhaust pipe that connects to the manifold and it's located immediately before the catalytic converter in a section of pipe that joins the separate pipes from each exhaust manifold. It's screwed in from the top side, so if you laying down on the ground trying to see it, you'll probably miss it.

Hope that helps
Dr. Loot's Assistant