2008 Hummer H3 wiring location

There is not a VSS circuit, it is all done through a communications buss. I don't dabble in aftermarket audio, but I have seen some installations that have an interface module installed, that may be what you need. I would quiz an aftermarket installer, and see if they can provide you with some information. The factory schematic is the same for radio, or nav, and it has the buss circuit involved. The base system probably uses speed info for speed dependant volume, whereas NAV would use it for that, and GPS calculations.