It's possible that you may have a positive terminal under the hood. In my 97 E420, the battery was under the rear seat. Under the hood on the driver's side was a post that was connected to the positive terminal of the battery. It was in a black housing with a red + on top of the cover.

__________________
Lloyd

[89 300TE]
[97 E420]
91 300 CE
95 E320 T