what model mercury do you have?
I looked at the technical service bulletins for your milan, and there are no common problems with the restraint sytem.
The air bag light comes on when the supplemental restraint system detects a fault in the system, the air bags will NOT deploy while the system detects a problem.
I would take your milan to a dealershop friday if they are open, or monday (i am assuming your milan is under warranty).
the only way to diagnose this problem is with a ford dealership scan tool.
below is the diagnostic procedures the dealership must perform to determine exactly what is causing the concern in the restraint system.
Diagnosing Customer Concerns With On-Demand DTCs
If the air bag warning indicator is reported ON by the customer when the vehicle comes in for service, connect the scan tool and follow the Diagnostic Instruction procedures in this section to identify the concern.
Once the DTC is known, read the Normal Operation of the pinpoint test for the DTC involved.
Using the scan tool with the use of PIDs and active command(s) may be of assistance in diagnosing the concern.
i hope this helps