My car is a 2000 S. The magnetic switch actually fits inside a cavity on the side of the plastic vent valve housing where it lives pressed against the fill tube in the right spot to detect the flapper. The whole vent valve just clamps around the fill pipe up at the fuel door. When I finally got my vent valve and switch back together correctly I could hear a faint click and hear the tank breath out when I pressed the flapper back with my finger.
I haven't seen the filler tube in the tank but I've read a technical article that says there is a "spit back" valve on the fill tube. I think it keeps fuel from splashing out when the tank gets full. Here's a screen shot of a diagram I found.
