My light works sometimes I have notice some time when it rains it hold water
interior light leaks has there been recall on 2004 rangers with that problem. on 2004 Ford Ranger
Do you have a cargo light ? Is the interior light the one close to the cargo light. If so maybe bad seal at cargo or 3rd brake light.
It's the third brake light at the top of the back side of the cab. I heard they had recalls. But my ven# was not one for the recall.
Just take the screws out holding the light to the cab, get some clear silicone, clean the area under side and apply a bead of silicone, remount the fixture and let it cure. Wa La no rain inside.