How to open boot (trunk)

My friend has a 180E year 2001. His battery is flat and so he cannot unlock the boot (trunk) with the key fob. Although the metal key will open his
driver door and the glove compartment it will not turn to open the boot.
This is the only metal key he has and I'm wondering if this could be a "service metal key" which will not unlock the boot?
Could I insert 12V into the electric lock system by just jumping from another vehicle to any +/- point on the car?
Beemer
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There is a remote battery connect in engine compartment... Positive is in a black case with big label on the top. and negative is a brass stub nearby on the body. Just hook up the jumper cable and you can get the remote to work... no need to jump start car.
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