MACA probably spot on but if you're not up to speed on Rangies yet you might need a bit more of a clue.
Unlike almost every other car there isn't a little black plunger that gets pushed in when you close the door and switches the interior light off. In a P38 there's a microswitch on the door mechanism, inside the door. If this has come loose or is falling apart then you get the result that you have.
Don't ignore it, it'll only get worse, and it's also part of the alarm. If it's going to fail totally what's the betting that it'l be in the wee small hours one morning.