I did exactly that a few weeks ago on my 06 4.8 and everything worked right out of the box. However, I don't know if that works on other models. Note that I had bought the replacement loom, not the retrofit loom. CHeck my posting on the Nav forum and you will see pictures of the loom.

Also, I had tested the Nav on my 03 before hooking it to my 06 so I knew that part was serviceable. Don't know if you have an opportunity to test the navcomp on another car to verify that it works? The GPS antenna is irrelevant for the test.