Item #6 is actually a spring clip. it is real small and can be overlooked. It holds the brake light switch (#8) firmly in place so it does not flop around.
#6 is easily overlooked or discarded when dismantling the right control and can be dropped without noticing it when replacing the switch.
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The brake lever pushes on the switch button until you apply the brakes. This releases the button and the brakes light. If this spring clip is not properly in place the brake light switch is flopping around and the lever can not hold it compressed.