Push Brake Pedal and the instrument cluster illuminates Why.

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Hi I've got my series 1 vn commodore sedan and just the other day I've noticed that when I engage the brake pedal the instrument cluster illuminates lights up why?

I first noticed this reversing out of my shed with the car, I've checked the rear bulbs and replaced them etc. also looked at the earth's in the engine bay etc. not sure what else is there a earth in the boot or something, can anyone suggest where I should look.
Cheers
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[greenacc]

Any work been done on the back end of the car at all recently?

 

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No nothing recently. I've just spray painted it external boot area plus rest of car did not take anything out the only earth I've removed and put back was the earth on the passenger side outside under the wiper blade motor area under the water deflector from the main front windscreen.
cheers
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[MikeC]

As I see it for my VQ , the tail lights earth through the BCM. The BCM earth is the one near the battery. I'd give it a clean.