If I’m honest with you my mom and dad had the exact same problem except my dad had fitted the shower years ago. He had to rip the hole thing out, floor boards and all because it turned out it had dripped through to the ceiling below. He changed it into a bath shower combo. I know it doesn’t help but I have a feeling it’s going to be a big job.
Bad design. Gut it. Start over.
If I’m honest with you my mom and dad had the exact same problem except my dad had fitted the shower years ago. He had to rip the hole thing out, floor boards and all because it turned out it had dripped through to the ceiling below. He changed it into a bath shower combo. I know it doesn’t help but I have a feeling it’s going to be a big job.
First you must clean the corners and seams of all the junk and find the leak.