on a corsa the inside light switch is part of the headlight switch right ? you pull it in and out ? tho Im not sure why you want that one... isn;t the one you want the one that turns it on when the door is open ?
if so... its switched on/off by the negative not the positive, look for the little push switch around the door frame somewhere... when you open the door, switch pops out, contact connects to car body and bingo theres your negative
for the headlight ones... take it from the back of the headlight itself (easy, but not so tidy) or the back of the fuse box (little harder, tidier tho)
edit: p.s. if you need a +ve from the interior light for your sircut to work you could easily wire up a small relay with a perm positive and have the neg powering it on/off....... tho i guess if your building the circut above etc.. then you know what your doing