Here is the text from the 2003 Navigator Workshop manual.
Should be the same for 2004.
The Pin-typ retainer is in the center console
Audio Unit 
Removal and Installation 
Vehicles with column shift 
Remove the instrument cluster finish panel. For additional information, refer to Section 501-12 . 
Vehicles with floor shift 
Remove the instrument cluster center finish panel. For additional information, refer to Section 501-12 . 
All vehicles 
Remove the bolts. 
NOTE: Standard audio is shown, others are similar 
Disconnect the audio unit antenna lead-in cable and the audio unit electrical connectors. 
To install, reverse the removal procedure
THIS IS THE REFERENCED SECTION 501-12......
Instrument Cluster Center Finish Panel 
Removal and Installation 
Apply the parking brake and move the shift lever to NEUTRAL. 
Disconnect the battery. For additional information, refer to Section 414-01 . 
Remove the pin-type retainer and the window control switch panel. 
Disconnect the electrical connectors. 
Position the shift indicator bezel aside. 
Remove the screws and the center console finish panel. 
Disconnect the electrical connectors. 
Remove the instrument panel center finish panel. 
Remove the screws. 
Pull out on the panel under the air vents to disengage the clips. 
Disconnect the electrical connectors. 
To install, reverse the removal procedure.