Hi,
I just took a look at my door. I'm afraid it might be different because mine are powered but in the hopes that there might be similarities:
I was able to find a little black knobby thing towards the back of the van where the door contacts the van when it is closed. It is on the van itself, (as opposed to being on the door,) forward of the rear u-shaped "latch" (for lack of a better word, it is chrome colored and clearly visible if you have the door opened. It is apparent that the door latches onto this in the closed position so as not to open.) This black knobby device seems to have a spring mechanism in it and if you push it in with your finger it actually deactivates the interior lighting. Compare with the other door and see if it is working the same. If not then it may just be the culprit! Seems like you can just unscrew it and replace it with another one if you can get a screw driver to fit there. If both sides work the same then you'd have to look for some other functional difference between the two. Best of luck!