It is an over-voltage protection "Relay" with a stand-alone fuse on top, that is particularly important during engine startup.

Unfortunately, they are notorious for compensating/becomming less effective with age, and can contribute to rough idle/starts in an effort to continue providing the protection.

Do a search, and you will find .. it is one of the few cases you will have techs recommend replacement, without ever hooked your SL to a diagnostic.

If yours is original/old - replacing the OVP may reduce wear and improve starts, even if you have minor trouble now.