On my auto tailgate, I have to hold it down for 2 seconds and release, then it will open. The timing is important because if you hold it too long, it will trigger the alarm. If you hold it too short, it was too short. I actually count to about 2.5 in my head... it's that troublesome.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr