The 2006 Mercedes E350’s battery location is the trunk of the car, on the passenger side. You’ll find it right next to your E350’s spare tire.
To view the battery, you’ll need to first lift up the lower trunk cover using the clasp. You can lift the clasp and attach it to the upper rim of the trunk to keep it propped open. After that, you’ll need to lift out and remove the molded trunk carpet panel—and you’ll see your spare tire and your battery.