Someone heavy enough has to sit in the seat. The seat cushion has something called an occupant clssification sensor built in. The has to be occupied by a passenger, who is heavy enough before the air bag will activate. I believe the weight threshold is 60 lbs, but I am not 100% sure about the weight. The reason is that, Ford does not want a child to get killed or seriously hurt by the airbag. If the airbag deploys, a child would be at the right height to get their neck broken. The secondary reason, is reduced insurance repair cost. If there is nobody in the seat, why blow off a $500.00 airbag? So, with the key on, put an adult in the seat and wait a couple of seconds. If the airbag still does not activate, then there is a problem.
near the firewall on the passenger side
Yes , it's behind the trunk liner on the passenger side , look for an access hole ( or if it is a wagon , its behind the service panel on the passenger side of the cargo area )
The ECM in a 1999 Mercury Sable is located on the inside of the vehicle on the passenger side. It is located under the kick plate.
On the top of firewall on passenger side of engine bay.
The firewall is the metal " wall " that divides the engine compartment from the passenger compartment
The 1989 Mercury Sable 3.0 liter engine EGR valve is located on the firewall. The valve will be on the passenger side of the firewall.
The 1999 Mercury Sable computer module can be located behind the passenger side dashboard. The dashboard will need to be removed in order to view the computer module.
In a 1999 Mercury Sable the fuel pump shut off switch ( inertia switch ) is located behind the trunk liner , on the right side ( passenger side ) There is an access hole
Check under the front bumper area on the passenger side.
In the trunk on the passenger side under the trim/carpet.
I found the low pressure ac port on my 2001 sable by the passenger side firewall.
According to the 1999 Mercury Sable Owner Guide : The 6 disc CD changer is in the trunk ( or if it is a wagon , the passenger side storage compartment in the cargo area )