This is an unfortunate problem. The trunk can be a particularly difficult area to diagnose and resolve.
Water may enter:
1. Around the main seal.
2. Around the CHMSL lens.
3. Around the threaded studs that hold the rear aplique on.
4. Around the plastic clips that hold the top corners of the rear applique on.
5. Around the tops of the rear shock towers.
6. Around the tail light studs.
7. Around the lock cylinder.
8. Around the studs that secure the rear spoiler to the trunk lid.
9. Around the fuel tank door lock solenoid (if equipped).
and so on...
There is still some getting into my car that I haven't been able to track down yet
There is a TSB that specifically refers to the CHMSL housing among others.