On the corsa c, there is a section of bulkhead on drivers side which is a bolt in pedal box assembly, held in with 13mm head size bolts and sealer, the sealer goes hard with age and cracks, which in turn allow rain water into the car, so the fix is once all stripped down to gain access, spend some time and dig out all old black sealer, make sure all bolts holding pedal box in are tight, then reseal like u would reseal a bath