... and can you still use the Nav?

This is not and easy or cheap thing to do. There are a lot of threads on this subject already so I would suggest that you search for them and read about it.

Short answer is with NAV not so easy as the nave unit sit right were the new head unit would need to go and is large enough to not allow another head unit in that space. The OEM radio and NAV section are all made into one double high unit and as such has not place else to be placed.

If you did not have NAV it would be much easier.

Best bet is to put the aftermarket head unit in the secret compartment with will fit an aftermarket radio. Again searching you will find a few people who have done that.