As said start at lever or rather the switch, it merely completes a circuit by going to ground, the other side of the bulb being a live, from ignition switched supply, so you either have an earth missing(handbrake side) or a live missing other side of bulb holder, maybe take bulb out and do a continuity test between lever and bulb holder, occasionally it has been known for the wire to get trapped by lever and broken.
tabetha