WARNING: This page is about a different car, the 2010 BMW 550i, 2009 BMW 550i, and 2008 BMW 550i. However, it is still accessible from the selected car via links, so may be relevant.
CARTRIDGE GUN 81 43 2 159 881/883
Fig 1: Identifying Cartridge Gun Speed Controller
Courtesy of BMW OF NORTH AMERICA, INC.
IMPORTANT:
Set speed control (1) to interval stage 3 to prevent overloading the cartridge gun.