OK, I understand the original question, and agree that nobody knows how to put a timer on the chime. Don't think it is possible. Personally, I would use the seatbelt even to back into the garage.

But my question is, why is it necessary to get out of the vehicle several times when backing up, given that you have large side mirrors with remote control on them, and the PDC. It just seems like major overkill. Commercial vehicle driver training here used to require getting out of the vehicle every few hundred feet when reversing, but proper use of mirrors and a single check for small children before starting to back up should be sufficient for anything like entering a garage.

If you are uncomfortable with the size of the vehicle in relation to the parking space, practicing backing up into tight spaces would be less trouble than getting out all the time.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue