This was taken from user "schramautoparts". In no way is this my link, or writeup.
here's a link to his ebay store: eBay My World - schramautoparts Applies to Chevy Silverado, Chevy Avalanche, Chevy Suburban,
Chevy Tahoe, GMC Sierra, GMC Yukon, and GMC Yukon XL
The following are instructions on how to install a factory navigation system into a full size General Motors truck or SUV. These vehicles include the Chevy Silverado, Chevy Avalanche, Chevy Suburban, Chevy Tahoe, GMC Sierra, GMC Yukon, and GMC Yukon XL. In this example we installed the navigation system into a 2008 GMC Sierra 2500HD that did not have the navigation option from the factory.
In order to install the navigation unit you will need the unit itself, the navigation antenna, and the navigation map DVD. In this particular case we used a GM navigation unit which was OEM part number 15940102 with a GM antenna which was OEM part number 15201798. Below are the steps necessary to install this OEM navigation system into a vehicle that did not originally have one. Navigation Systems Available Here!
Remove the two ten millimeter bolts that hold the oh sh#% handle on passenger side A-pillar. Remove the handle. Next remove the A-pillar trim panel. The trim panel snaps on with a couple clips. You will also need to unplug the tweeter mounted to the trim panel.
Remove the A-pillar trim on the drivers side. You will need to remove a seven millimeter bolt located underneath a small plastic cap. This trim panel also clips in with a couple clips. Again you will have to unplug the tweater mounted in the panel.
Remove the dash trim panel that buts up against the windshield. This part clips on, there are no bolts to attach it. Be careful as the part is made of plastic. You will have to disconnect the light sensor that is connected to this trim panel. To disconnect it just turn the sensor counter clockwise and it will come out.
Next remove the trim plate around the radio. This part clips on. You will not need to remove any bolts or nuts to remove this panel. You will have to disconnect the wires for the heater controls.
For a good practice we are going to test the navigation unit before going any further. Remove the old radio and the heat control unit (which must be removed before the radio). Make sure the key is off on the vehicle. Plug the antenna into the navigation system and set it on the dash. Plug the radio wires into the navigation unit. You will see the navigation system has 4 inputs on the back for a wiring harness to go into. You will find you only have two wiring harnesses available. Do not be alarmed, this is correct and you cannot plug the wires into the wrong place. The plugs are slotted to ensure that the correct wiring goes into the right plug. Once you are all plugged in (don't forget about the am/fm antenna!) go ahead and turn the key on. Test the unit to make sure everything is working properly. You will not be able to use the navigation until you have navigation disk. It will however tell you your current GPS coordinates. Keep in mind if you are inside of a garage or around any large building the antenna may not be able to communicate with the satellites. If the navigation unit is functioning properly then please move on to the next step.
Now you will need to fish the navigation antenna through the dash. I was able to fish the wire through the dash with my hands. You may find you need a coat hanger and electrical tape to get the wire through properly. Make sure that the antenna wiring does not get pinched when you install the navigation unit into the dash.
Snap the antenna into place. There is a pre-installed location for the antenna to snap in directly underneath the dash/windshield trim panel that you removed earlier. The snap in location is near the center of the dash off set to the passenger side a few inches.
Step 9: So there you have it. How to install a factory navigation system into your Chevy Silverado, Chevy Avalanche, Chevy Suburban, Chevy Tahoe, GMC Sierra, GMC Yukon, and GMC Yukon XL when it did not come with the navigation system from the factory!
If everything works properly you should be up and running at this point! When the unit first turns on you should be greeted with the logo from the make of your vehicle (in this case GMC). The nav will need a navigation map disk to function as a navigating system but you will be able to use the the radio and all of the other features of the navigation unit. The map disk is inserted behind the screen of the navigation system. The screen folds down and the navigation disk is inserted. This allows you to use the other visible disk drive on the system for CD's, MP3 disks, and even DVD movies (which can only be viewed while in park and only available on units with DVD movie capability). You will also want to disable the rear camera option (since your vehicle did not come with navigation it also did not come with a rear camera) or else you will get an error message about your rear video system. To do this go to"menu", click "display", choose "rear camera options" and uncheck the box to view rear camera while in reverse. This will eliminate the error message and your nav system will remain the same when you shift into reverse.