On our '98 E320, the battery is under the back seat. However, there is a connection that can be used for boosting etc under the hood on the driver' side - I think that it has a red cover that hinges up to expose the lug.

It is not so easy to find a ground connection because the engine has a complete plastic cover - The manual says to connect to an engine lifting lug near that back of the engine.

Read your manual under boosting - it should show these connections.