The cool thing is, the extra button could be put on your MP3 player, so you can mount your MP3 when your not busy on your phone. While there are screw mounts available, I've only used double sided tape to mount this to my dash and it has never fallen off.
And, if you ever need to transfer to a new vehicle or move the button to a new phone, the tape below is the best!
I use the dash holder I listed above. PLUS, I use THIS ADAPTER, along with a regular mp3 cable and it pumps the TelaNav directions through my stereo speaker. Makes an AWESOME GPS setup!