The 2003 Chevrolet Impala doesn't have a single, easily identifiable "firewall" component in the way some might think of a physical barrier. The term "firewall" in automotive contexts usually refers to the
bulkhead separating the engine compartment from the passenger compartment. This is a large, complex assembly made of steel and other materials. It's not a single, easily-removed part. You won't find it labelled as "firewall."
To be more precise about where you need to access a specific location behind the firewall (for example, to install something), you'll need to specify what you are trying to do.
