Yellow and white wire goes from the handbrake handle switch up to behind the dash. The circuit looks for a ground, so when you pull the handbrake the switch gives the circuit a ground and the lamp comes on.
There is a Lucar connector just a ways away from the switch and that can corrode, check that.
Also check the switch again, they are troublesome, but you can take them apart and clean them up.
Ground the wire manually at the seatbox. If the light comes on you know it is the switch.
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