Orient the negative n image as it sits in the film back. Then try to figure where the light is entering... dark slide, hinge, latch, face plate screws lose or missing, rev back not seated properly... etc.

Now take a good look at your seals.. gummy, missing, dry n cracked?

Then there is the final test... the bright light inside the camera trick, done in a dark room. If you can have that much light get in, you will certainly see it coming out.