Look here:
http://www.volvospeed.com/volvo_performance.php
Check "modifications"
look for aquamist. There's a detailed (with pictures) description on how to install an aquamist in a volvo 850!.
Should be helpfull info for other kits as well.
They use the open port of the vacuum tree.
I don't really understand what water comes near the switch?
Janis