2/0 high quality cable/wire from the (-) battery lug, directly to the engine block. from that point, fan off of that to your solid copper ground bar/strap for all of your negative connections.

I prefer 2/0 Anchor copper cable. You can buy it from West Marine, however make sure to have the battery lugs crimped on with a hydraulic lug crimper, and use the Anchor heat shrink.

Since current flows from negative to positive, your ground/negative cable is very important. Also, ground the battery to the frame in the trunk as well.

I also prefer to use the Ford style solenoid to attach my Positive cable from the battery switch to this solenoid. That also includes a 2AWG from the main positive on the solenoid lug to your Positive junction point for all of your electronics. Making sure that the junction his also high quality Copper.

Good luck...Mark