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Hi Sarah - as far as I know the brake light system and the fog ilght switches are independent - I would still favour the short or poor earth in the rear lamps. If nothing is self evident around the rear lamp clusters you may need a multi-meter to test for continuity. What normally happens is that the current going to brake light bulb can't find an earth so uses another cable as a means of contact and in the process any bulb on that circuit will illuminate.
Currently in madnoel10's garage:
Honda Civic 1.4l