Factory installed Nav issue, no vss signal?

[amighjm6]

08 Tahoe with (to my knowledge) factory installed DVD and Nav head unit. Bought the vehicle used, so not sure if it has ever worked...but the nav isn't providing turn by turn directions or any kind of guidance. if i input an address, it will plot the course, and highlight the path i should follow, but that's it...no turn by turn, and no recalculation if i get off course. i get the "you have reached your destination" prompt when i arrive, and it seems to track me well on the map. i've been researching for over a month, i've pulled the radio and checked all connections and am at a loss. it sounds just like the issue people are having when installing the Nav unit after the fact and are not hooking up the VSS signal, is it possible my factory installed unit is not receiving it either? how can verify that's the issue? not sure if this is relevant or not, but the nav unit only has 2 harnesses connected in the back, the 2 closest to passenger side i believe, the 2 connections near the driver side are empty and there are no wire bundles there to attach to them...i assume these should be empty and are used for a feature i don't have? the dealer says that's the way it should be, and wants $98 an hour to look at it, and most likely won't be able to do anything except restore it factory settings.
any help is appreciated,
thanks

---------- Post added at 01:04 AM ---------- Previous post was Yesterday at 11:59 PM ----------

solved my own problem...apparently the Nav unit was not a factory option, so whoever installed it probably did not run the VSS as they should have.

want to know which options your car came with? check this site...pretty cool:
http://www.compnine.com/vid.php
and punch in your vin#