My wife was driving our '04 X5 4.4 two weeks ago and was rear ended by a young woman who had no valid license and, of course, no insurance (got to love SoCal!). She did have a cell phone though and she was probably talking on it when she hit our X (figures). Anyways, the damage sounds exactly like yours...dented lower tailgate, crushed bumper, broken tail light, bent mufflers. Yes, the power cut-off kicked in and my wife couldn't start the engine either. AAA took it away on a flat bed (I always told my wife to ask for a flat bed to risk the least amount of towing damage). We should have ours back next week, and I'm taking it to the dealer to have it checked out afterwards, just in case, to make sure all the electronics, etc. are OK. At least the X did what it was supposed to do and no one was hurt. I told her that she needed the extra safety features of the BMW, and now she really believes me! Good luck w/your repairs.

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'06 X5 4.8is, TiAg/black, cold weather pkg., comfort seats
'07 750Li, TiAg/black, sport pkg.
'05 Ford F350 silver met./slate gray, Lariat, 6.0 PSD, CC, DRW, FX4