Citroen Xsara 99 inside light only works with direction lights, any clue?

[Electranieda]

I bought a 99 Citroen Xsara. The inside light, the one in the front side that usually turns on when you open the door, only works if you set it to "on" and you turn any indicator on, either left or right, and gives an intermitent light similar to the indicators. How come?