05 navigator sub amp

Hi everyone, I am new to the forum and looking for some help.  I am getting ready to put an AVIC-Z1 navigation unit into a 05 Navigator and I will be replacing all of the door speakers and adding a new 4 channel amp.  The customer is ok with loosing the factory sub and does not want to add an aftermarket sub.  However, if at all possible I would like to keep the factory sub working but I have never tried this and any help that anyone could give me would be greatly appreciated.

Metra 70-5521. I can't seem to picture the radio connectors on the back of the navigator, but this is the harness for retaining the sub on newer ford/lincoln/merc. Mike

thank you very much.