Like your Mustang's factory wiring, the Painless system disconnects at key locations, like the firewall. The main wiring loom under the dash has a female multiplex connector at the firewall. This is where the firewall-to-headlight loom connects. The firewall-to-headlamp loom handles all underhood power. It ties into the voltage regulator, headlamps and parking lamps, horns, starter solenoid, and alternator/generator.