I bought my car used and had this same trouble. I took out the unit (two screws) and had a look at it. There are metal strips that carry the power to the bulbs and they were not making good contact. Bend them a little and it should fix it. BTW, I latter installed led bulbs and now when I roll a window up or down the lights flick on but only when I hit the switch. Others have this same thing happen with led bulbs.