Hi, Steve, The gap is to let out light. If you have a original style switch it will have a place for a bulb holder on the back. ( So you can find the switch in the dark) The bulb is on the instrument light circuit. On which hole to use try one then the other then both. If I remember right, you should feel or hear a lite click when the wire is pushed in. I installed a new tumbler about 8 years ago. Have fun, Brian
