WARNING: This page is about a different car, the 2002 BMW M Roadster, 2002 BMW M Coupe, 2001 BMW M Roadster, and 2001 BMW M Coupe. However, it is still accessible from the selected car via links, so may be relevant.
Unfasten cap on expansion tank.
Fig 1: Locating Cap On Expansion Tank
Courtesy of BMW OF NORTH AMERICA, INC.
Remove front underbody protection and unfasten drain plug at bottom of radiator if necessary.