Preparatory work:

WARNING: This page does not describe the selected car, but rather 16 other vehicles, including the 2006 BMW 330Ci, 2005 BMW 330xi, 2005 BMW 330i, 2005 BMW 330Ci, and 2004 BMW 330xi. 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: Covering Working Area Over Large Area At Outlet Of Wire Starter With Fabric Adhesive Tape
Courtesy of BMW OF NORTH AMERICA, INC.