Can someone clear this up for me?

I have an 05 with stock nav. I would like to upgrade to a DD HU. I've heard that once you upgrade to an after market unit, you lose the voice navigation on the stock nav. Is this true? If it is, is there a way to wire it so you have both?

Thanks

 

Check my350z.com's audio section and there is a DIY on wiring so you still have factory nav voice.

 

Thanks!