Maybe approach the problem at a different angle. Use the same power feed but install a relay to open the existing circuit when the key is off. Not sure though what effect this would have on the TCU operation, it might need to have power all the time or maybe need power before the key is turned on.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)