Found a really good free programme that someone has developed.
Its called BlueGPS4droid. It allows you to connect external devices in any programme from Maps to Navigation.
There is no trial or time limit and it just works.
Don't know if its in the market but I found it here post #19
http://forum.xda-developers.com/show....php?p=8158118

Direct download link
http://sourceforge.net/p/bluegps4droid/home/

I accept no responsibility for this software, developer is distributing as freeware but usual warnings about using external software ect.

Just download .apk save to dir on N900 /home/user/MyDocs/extras is a good place.
In NitDroid download any file manager from Market I use OIFilemanager.
Navigate to extras folder.Click and install.
Remember to allow external applications to be installed in settings.
Enjoy Google Maps with voice guidance for free.
Writing this on N900 spelling mistakes ect.