stereo wiring for 2003 expedition eb

I'am trying to find a wiring diagram for the audiophile sound system in a 2003 Expedition Eddie Bauer. It does not have the rear entertainment center or navigation but does have the rear audio controls. The stereo has three plugs that go in the back, the center one is a 20 pin unit and has, I think, the radio mute wire. I need to know which wire is the mute wire.
I put in an aftermarket stereo and the 20 pin connector is not used anymore though the new steering wheel control is attached to one of its wires. My problem is that the parking aid does not mute the radio anymore if it senses an obstacle when I backup. The new radio has a mute wire but I do not know where to attach it. Thank you.

Regarding my request for a wiring diagram for the mute wire on the 2003 Expedition Eddie Bauer, I now know that it is the dark blue wire on the center plug of the factory radio.

What color is the mute wire of the aftermarket radio?  Have you used a meter to see if the vehicle's mute wire goes to ground when the radio should mute?

Thank you for your reply. I have determined that the mute wire was the dark blue one in the factory harness.