Preparatory work:

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.

To avoid paintwork damage, tape off working area over large area at outlet of wire starter (2) with fabric adhesive tape (1)
Cut off required length (A) from cutting wire
(A) = Window diagonal x 4
Check whether wire can get caught on rivets or retainers; if necessary, insert guide needles in bonding joint and guide cutting wire past

Fig 1: Identifying Outlet Of Wire Starter
Courtesy of BMW OF NORTH AMERICA, INC.