The interior light is grounded thru the switch touching the back of the dash metal. It has a tiny little metal bar that touches the base of the switch when you turn it to on. If you cant feel the tiny button on the top of the switch to pull the stem out of it w/knob you can take off your knob with an allen wrench.
After removing the knob then you unscrew the nut holding the entire switch onto the dash. The switch should have enough length of wire to pull it down to look at it. Take a small bit of scotch brite and clean the back of the dash where it touches to get the ground.
