As mentioned the "doghouse" was there for the head end brakeman when the loco cab was too small for a third seat. His job when the train was moving was to watch for any problems that he could see such as a smoking journal (hot box) or minor derailment or whatever - dragging equipment.
so whether the doghouse had a light or not, it would be off most of the time and especially at night so the brakeman could observe the train.
My opinion is to skip the light, but glaze the windows and put a crewman in there.
Another note, I read someplace a while back that many if not all pennsy doghouses had steam heat pipe in.