external voice

I have a nexus 4g and the carrier is sprint. I use the navigation app quite a lot and would like to know if there is a way to link the voice to the earphone jack. As is its limited to the speaker in the phone. I thought maybe some one would know od an app to replace the navigation app that came with the phone and possibly that app would engage the earphone jack. Any suggestions??

I have never tired that but I'm more than happy to help, just let me confirm what your saying when you plug something into the headphone jack I.E external speaker or earphones it continues to come out of the phones speaker?
If your car has bluetooth you could try that as well?