Interior lights will not come on

[Joel.]

This is probably going to sound silly, but I for the life of me cannot get the interior lights on my LW MK2 to come on when the car is unlocked and a door is opened. I remember they did work, but I turned them off a couple of years ago when I had a weak battery. Now I just can't work it out, and the manual didn't help me.
I have tried holding the "off" button, tried holding the button next to it, tried short pressing both but nothing seems to work.
What's the trick?