Battery in the trunk.

The bigger the cable = LESS resistance

The ground cable should be the same size as the pos bat cable. You can ground it anywhere, preferably an area where the metal is thicker, like the frame or where the frame is attached to the trunk floor. Either way make sure you crimp and/or solder the terminals good and clean the paint off the area where the grounds attach.

Don’t forget to use an equally sized ground from the engine to the frame.