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It's one of those generic things that the Japs specialise in confusing you with.
When you press it, it will check your rear lamps and if one of them is out (blown filament) then a lamp should light up, possibly on the dash somewhere, to tell you that a lamp is out - which could be a tail light, brake light or indicator.
The switch may also check the water level in the radiator and show a fault lamp for that as well should it be low. There is a good possibility that it isn't even working.
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