Great advice, but I've already replaced all gasket
Thanks everyone. It looks like old BMWs leak in the trunk! I've already tried all the normal suspects:
1. New rear window gasket
2. New gaskets around the tail lights and the side marker lights
3. New trunk gasket.
The latter is trouble because I've had rust trouble under the gasket. I've had to pull up sections, scrape the rust, use phosphate primer, paint, and then reseal the gasket. I'm probably just not gluing it down right. I'll keep checking.