Genuine OEM Interior Dome Light Assembly. Originally manufactured by Hella. This liht  is installed in the upper cabin headliner as the main interior dome light, or integrated into the C-pillars / side headliner areas depending on the model. On models equipped with a check panel or a manual/electric sunroof overhead panel, the light snaps right into the trim directly above the driver's right shoulder/rearview mirror area.
On slick-top (non-sunroof) cars and certain 4-door sedan/coupe configurations, this exact rectangular assembly dual-purposes as the rear illumination unit pressed flat into the left and right C-pillars near the rear windshield.
The light housing does not screw directly into the metal; it relies on a hidden plastic structural bracket nested right behind the headliner fabric.
Note: It is generally not used in the E30 convertible models, which required a different lighting assembly layout.