I replaced 4 in my '01 (since retired). The ironic thing was that it wasn't 4 as in all 4 windows, the driver door broke twice. I started at the driver door, then 3 months later did the passenger rear, then 6mon for the driver rear, and then back to the driver front in another 4 mon. Got quite quick at it...

I sold the car to a girl who works at my office 2 years ago with 95k miles. She has since put another 15k on it (drives very little) and has had no reg problems (fortunately).

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2005 ///M3 Competition / Interlagos Blue-Black /6spd/SS Race Exhaust/Angel Eyes/Passport HW/Mich. Super Sports Soles (love em)
2008 ///M3 Melb Red w/ Beige Novillo, Tech, Prem, & 19" (Retired)
2005 X5 4.4i Estoril/Grey Sport, Nav, DSP, Prem, Pano, etc. (Wifey's)
2007 335i Silver/Black SP,PP,CWP, Comfort access, & Sirius (Retired)
2001 X5 3.0i (Retired) Silver/Gray Sport w/19"/Prem/DSP/CWP
2002 YZF600R (Retired)