I don't know the path of the sunroof drain on an 850, but if I recall correctly on my 240 it followed the pillar forward and drained out into the front wheel well area. I imagine it might be the same for the 850. You could try cleaning it with stiff wire like a coat hanger.

The leak might also be due to a torn interior door window plastic seal. Have you ever taken the door panel off in your car? You might have damaged it when you did. Having water on the rear floor makes me suspicious of that possibility. Check the bottom of the door panels for dampness. If wet that would confirm an interior doop panel leak. Also check the small drains at the bottom of the door to make sure they aren't clogged...
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Bessie - 95 855 GLT Sportwagon; 93 854 GLT; Inga - 90 244 DL 300k, gone but not forgotton