Installing an aftermarket amplifier in the boot of my mum's 2003 getz. Everything seems to be going smoothly. Amp is in, new speakers installed in doors, 16ga cables all installed, new head unit and a little active sub under the driver's seat. However one thing's got me.....

Grounding. Most of the metal inside the car has no continuity to the battery negative, nor to each other.

Where's the easiest place to ground the amp going to be? If I can avoid running another cable from the engine bay I will. Joining all the panels together with ground cables is an option but that's a lot of points of failure.

Cheers!
-Eiac