Passenger SRS only works below freezing

When the temperature is above freezing, the Passenger Airbag light is always "Airbag Off" and the flashing red light does NOT engage, regardless of whether a passenger is sitting in the seat and regardless of whether the seat belt is latched.

When the temperature dips below freezing, the system works as expected:

• No passenger: light is "Airbag"
• Passenger with seat belt: light is "Airbag On"
• Passenger with no seat belt: light is "Airbag On" with red flashing seat belt sign

I can't imagine the issue is related to a component inside the passenger compartment (the seat weight or latch sensors), as the temperature inside the vehicle is usually above 60 F. That said, are there any components outside the compartment (under the car or in the engine compartment) that could be causing this behavior? Is this a water freezing/unfreezing issue, caught in some wiring somewhere?