The battery's in the trunk, what's under the hood is just a fuse box and terminals to jump the car.
Since the battery is under the quarter panel by the driver's side tire, there is your positive and negative terminals up top where you would hook the booster cables to to jump the car.
Maybe you need a jump start
The battery doesn't have a full charge, or may have a dead cell.
Yes
There are auxiliary terminals underneath the hood.
yes
clean the battery terminals - strong wire brush or a little baking soda & water.
No, it is not possible to jump-start a car battery with a positive-negative connection. Jump-starting a car requires connecting the positive terminals of both batteries and the negative terminals of both batteries. Connecting positive to negative directly can cause damage to the electrical system.
Where are the jump start terminals under the hood on a 2003 e320
The 86 Calais has a connector under the dash, driver side. You have to jump the top right two terminals, turn the key to on (don't start it) and count the flashes.
I would check the battery terminals, probably a buildup of corrosion between the posts and the terminals, clean them up with boiling water and sandpaper till both parts are shiny clean. If you are connecting the jumper leads to the battery terminals when jump starting and it works fine then this is the only possibility.