msymons - it will be a simply wiring issue. Use the wiring diagram and ensure that the nav sound is wired to the nav input of your system

You must have a new-gen radio module, because the old-gen share TV and nav audio on the same input, where as new-gen have a different input for nav sound

So simply connect the nav sound from the nav to the nav voice input of the radio

The wiring diagram says it all!