Remove cowl panel from passenger side and snake wire upward. It will end up in the proximity of the battery from the other side of the firewall. A very tight squeeze...especially with zero-gauge!
Wish I had better pics of this, but I documented a similar install on my former W124 using 4-gauge wire to the battery post:
Footwell removed. Routed power lead over to the right side:
Engine view with lead arriving at the battery:
Hope this helps.