The firewall on any car is the wall between the engine and the cab. It protects the driver and passengers from heat off of the engine and in case something goes wrong with your engine. Most parts won't be able to break through the firewall.
Should be firewall area by booster.
For a Mercury Sable V6 engine : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
routing for serpentine belt 2003 mercury sable
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.
On a Mercury Sable : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
firewall 1-----2-----3 4-----5-----6 front of Mercury Sable > driver
On a Mercury Sable , V6 : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
On a Mercury Sable V6 engine : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
near the firewall on the passenger side
On a Mercury Sable V6 engine : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
36R
On a 2003 Mercury Sable : The thread on cap on the engine coolant reservoir is the 16 PSI pressure cap / radiator cap