in the boot, if you peel back the carpet i would expect to see a lump of electrial wiring, and at least 1 cable (possibly green/yellow) connected via a crimp connector to the car body. this connector is one of the "earth" points. (the battery had 2 terminals a positive and a negative. The negative (on modern cars....) is connected to the car body....

its this point that for years has been issues with fords....you always used to see old fords indicating and brake lights dimming or flashing at the same time....

give this cable a wiggle whilst a mate presses down on the brake pedal, if this cures the problem slacken off the crimp connector screw and then re-tighen it...that should be enough to clean off any dirt...or a wire brush and some WD40 to do a proper job....

good luck

paul.