This module (also called the Comfort Module), controls several things, including the power windows and sliding/pop-up roof. It is under the left rear seat bottom. Pictures shown are from a 1992 300CE.

I had a severe battery drain (would flatten the battery within 2 or 3 days) which I could not trace. Even with all fuses pulled, was still getting the drain. I reset this relay (unplugged it for a few minutes), and the battery drain was gone. Coincidence?

Must have been a coincidence, as the battery drain came back. Drain was later determined to be internal to the Becker radio. It was intermittent and proved difficult to troubleshoot.