The shutter is in the lens, so it's easy to find the light leak. Open the back, go into a dark room, shine a flashlight round the outside of the bellows look inside the back for any pin-holes the light is getting through. Also, check the edges of the film back door to see if the light trap (folder that old, probably felt strips or just a light tight indent - but could be decayed foam, I'm not familiar with the golf 63) is making a good seal. As a cheap quick fix, you can put black electrical tape over any pin holes in the bellow. It isn't a permanent fix, but using the camera like that for a few rolls will probably tell you if spending the money to send the camera away for a CLA / bellows replacement is worth it to you.