sorry, I was gettign the two issues at hand confused. There are two conditions:
1) the passenger wanrning light is on constantly, but the SRS Malfunction light (cluster) is not on.
2) The passenger warning light NEVER comes on, and the SRS malfunction light is on constantly.
The only things that I can think of that would cause #1 to happen would be something wrong with the weight sensor causing it to read between 9-40 lbs constantly, and something shorting inside the seatbelt socket, causing it to think that the seatbelt is plugged in with no one in the seat.
Here's a question for you, if someone sits in the passenger seat and puts their seatbelt on does the light go out? The next thing i would do is get a flashlight and look under the seat to see if any of the wiring or sensors are loose or damaged.
If there's a TSB out describing this condition and you're still under warranty than it seems like the easiest thing to do would be take it in and let mazda deal with it. Anyone have a link to the TSB?
Issue #2 was caused by the wrong bulb being installed and was fixed by installing the right wattage bulb.