in the case of that unit the on/off or I/O is misleading, rather than turning something on or off , if you turn the knob in the off direction it will remove the unit by releasing the actual module inside.
to return it to its connected state, simply set it in place and turn to the right and it will connect the relay and lock it in place. the relay inside is actually called a Mas relay and conrols several functions including fuel pump relay, air injection compressor and A/C compressor actuations. i believe the box with the clear lid that you refer to is the OV relay. what manual did you buy? most have a component locator for electrical items

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1967 250 S
1967 280SE
1968 250S
1971 280 SE
1973 220
1974 280 S
1975 450 SEL
1976 220 D
1979 300SD
1983 300 SD
1985 230 TE euro
1986 280SL
1986 500 SEL euro
1986 190E 2.3
1990 300 SE
2003 C230 K
2004 C230 K
1987 560SEL
1987 300SDL now in canada
2005 C230
1998 C43 AMG Sold to DON
1993 190E 2.6
300TD Former daily driver Totaled
current:
R320 CDI
Ram 2500 Cummins