If you mean the notek rear lights- the rectangular box with 4 square lights on the top half - then that was for use as a convoy light. The four squares had a green tinted glass in them. Each pair is set close together ,with a wider gap between the first pair and the last pair. If you were travelling at night and following the vehicle, you used the light to gauge how far away from it you were following - the 'Goldilocks" system. Too close and you saw the 4 individual lights, too far back and you saw only one blended right, but if you saw 2 lights (each pair merged) it was 'just right'.
The light fitting actually had a flap that could be raised or lowered. When lowered the convoy lights were visible. When raised the 4 lights were covered and two oval, red tinted brake lights were visible - used for day time travel. When the flap was lowered there was a small hole in the flap that was positioned over the centre of the right red light. When the brakes were applied the little red dot could be seen by the following driver informing him to slow down too.
From Squadron signals Stug 3 G Walkaround by Tom Cockle - an excellant low cost photo reference on this vehicle:
As Gino said, the front notek was to limit the amount of light visible too aircraft or other vehicles. The shroud on top prevented the source of the light from being seen from above.