Best location for Ground wire from amp

[x5e53]

Hi,

Where is the best place for a Ground wire in the trunk/boot of a BMW X5 e53?

There are lots of locations where i could put one, but was wondering what others have done before i start drilling.

[X5Sport]

Straight onto the battery as it's right there.  If not, then there should be somewhere by the battery where the -ve lead connects to the car.  I thought there were a couple of common ground points in the boot area once you get under the spare wheel.