What must be installed whenever wiring passes through a metal firewall, if not already present?

When wiring passes through a metal firewall, it is crucial to install a rubber or plastic grommet if one is not already present. This is because grommets serve as protective barriers that prevent the wiring from coming into direct contact with the sharp edges of the metal firewall.

Without the grommet, the insulation on the wiring could be damaged over time due to vibration or movement, leading to potential shorts, electrical failures, or even fire hazards. The grommet not only aids in preventing physical damage but also helps to seal the hole against moisture and contaminants, adding an extra layer of protection for the wiring.

While other options may have their uses in various contexts — such as metal couplings for connecting pipes, silicone sealants for sealing and waterproofing joints, and heat shields for protecting components from extreme temperatures — none are specifically designed to address the need for protection when wiring passes through metal enclosures like a firewall. Thus, the installation of a rubber or plastic grommet is the appropriate and necessary action to ensure the longevity and safety of electrical systems in vehicles.