I can't be more specific as I'm not more familiar with the newer cars, but you're likely looking for a module, usually labelled something like MAS. You will have to Google your specific year and model to find what it is and where it's at. It won't be a simple cube relay. The guts contain a circuit board and are known for poor solder joints and burnt traces.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]