What a headache but there's a way. Behind the clutch pedal too the left is a wiring port surrounded by a rubber boot. It is also to the right of where the hood release cable goes through the firewall.
Use a punch or long phillps screwdriver and make your hole for the wires.
Now, turn your front wheels all the way right and loosen the wheel well cover to where you can see and get your arm into the cavity.
Now hook your wires to a piece of metal coat hanger about 20 inches long, making the...

Wiring from cabin through firewall, 2006 Mustang