Why has my left rear turn signal stopped working.

What is your question?

I changed the bulb and checked the fuse. When I look at the old bulb it's not burned out. I'm wondering where the problem can be besides the fuse and/or bulb. There is a crack in the tail light and water somehow gets in the trunk (NOT A WHOLE LOT BUT IT IS THERE) when it rains. How can I diagnose the problem. I'm thinking maybe a harness. I'm trying to avoid trial and error by changing several parts. Any suggestions......ideas please.