Originally Posted by ILUVMILS

Make sure the vent tube is connected to the battery. Your car has an AGM, or "Absorbent Glass Mat" battery. It's also known as a VRLA, or "Valve Regulated Lead Acid" battery. These batteries don't give off the rotten-egg smell that most of us are familiar with. I've noticed a kind of funky smell in several 211's, that I've traced to the battery. I've been driving a 2004 E320 for the last few days (for an un-related issue) and I've noticed a slightly unpleasant smell every time I get in the car. I know it's the battery because of a 211 I worked on several months ago. As it turned out, the cars' electrical system was staying "awake", causing the battery to slightly dis-charge. The alternator was working overtime to re-charge the weak battery. After resolving the current draw issue, replacing the battery, and completely de-odorizing the interior, the smell never came back.

One more thing to keep in mind. Some early 211's are also prone to odors from the A/C evaporator, which don't shed condensation fast enough. Since you say that the odor is stronger when you open the trunk though, I'd look to the battery first.