It will make your drive easier, more relaxing in ways you can't imagine.
My wife has one, and even though women-hate-gizmos, she uses it every day!
It works ON ANY CAR newer than 2000.
Step 1: Parts list and schematic for Box labeled "2 Instructable"
Buy or build this board I copied from:
(they also give eagle-cad schematic).
The razor board has inputs:
The razor board outputs:
TTL levels when the audio is DTMF 0,1,...9,#,*
You will need two relays.
(radio shack Model: 275-001 or anything similar)