Thanks to tips from Chris and Alex, I removed the roof from my SP caboose, retrieved and glued in the errant window and restored the dead marker light to operation!
The marker light didn’t work because the plastic tab holding the contact grommets was broken in half (second hand caboose, somebody worked on it before). I glued the tab together with CA glue and added a small wood block to reinforce it. All the interior lights work; I’m satisfied with the way they look, so I don’t plan any modification.
Thanks again, guys!
John