Is this door the one that was repaired for rust - if so, could it be that they didn't put it back properly and you have water ingress where you shouldn't?
Could the sensor be affected by water ingress..? have you taken the door card off and see if it is wet or in a position where it might get wet?
I don't know how the sensor is positioned (I did take my door card of some time ago, but I don't remember the sensor's position..), but since the door is a 'wet' design i.e. water should run inside the door along both edges I would expect the sensor and cabling to be water-tight?
The car is now on it's 3rd airbag sensor.
Original - lasted about 2 years.
Replaced under warranty - lasted about 18 months.
Replaced - lasted about 3 months.
I have taken the door card off twice, one at TheBoys, everthing was dry and clean, we took the sensor off, cleaned up the contacts, then it came back on again within about 3 hours.
Bought a new sensor, still the inside of the door was dry and clean, replaced the sensor.
It's got to be something else.
TheBoy also cleaned up the multi-plug for the door electrics.
The only thing left is the connections behind the drivers kick panel.
Just reset the airbag light, tried it a few times, OK at the minute.