How to fix pants coil zipper that opens after pulling the cursor?

I've read the guide on how to fix a zipper that opens when the slider is pulled, but it requires that the two halves of the zipper can be separated.

I have the problem on a pair of trekking pants, and the two halves are kept together by the zipper box. The problem is the halves comes open right after the zipper box!

Is there any way I can fix it or should I replace the entire zipper?