That's the fuel pump relay. It has been superceded by part# A 003 545 01 05. That does not mean that your's needs replacing. Do a search on fuel pump relay. There's a way to jump two of the pins to run the fuel pump continuously.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino