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)
Step 2: Wire the DTMF/Audio
Since cellphone headsets are cheap, I would start with that
and cut the end and solder it right on the razor-audio jack.
Just solder it on the razor board, no need to buy yet another connector...
Step 3: Connect 5v to the razor board
So get a wall-wort.
E.g 110v to 5v DC converter.
You can get them at the Good-Will or Radio Shack
Use a voltmeter to verify its 5v DC, not AC.
Connect it to the 5v on the razor-board.
Step 4: Get the app
"Back Seat Driver, Pro".
It will cost you a dollar and 30cents of that goes to Google and 70 cents goes to the nonprofit called wikiSPEEDia.org
In the menu, click on "Cruise Control". Then click on ON, OFF.
You should see the LEDS on the Razor board light on.
If the LEDS don't change, adjust the volume-control on the razor board.
Step 5: Email me
Email me and make sure you have the latest.