I suspect someone rolled the lamp wires on the lighting board.
Here is the link to the LL website showing the board (#6 on the photo)
https://www.lionelsupport.com/...d4-b233-766e50ad2664
Getting into the docksider can be interesting...take a few photos with your phone as you pull parts off, so you will know how they all go back together. It's not as bad as the old K-line 0-4-0 Porter for repairs , but can still be a handful.