I found the schematic of the additional cicuit I added to the front interior light on the 300TE. I am going to make another for my 190E. It allows the light to fade to off at the end of the delay rather than suddenly switching off.
You need to connect a wire to the existing light unit between the switch (the position which operates with the doors) and the existing timer electronics. Bring this wire out to the additional circuit (preferably via a quick-connect in-line connector). I soldered the components to the transistor which is then mounted by way of its tab to the earthing screw on the roof frame visible with the interior light unit removed. The whole circuit apart from the transistor tab is sleeved in heatshrink tubing.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.