I would imagine its will be useless if you have the softop on or down.
I have a pioneer head deck with a remote mic that can be located just about any where in the car. It has a cord about 2 metres long. It also comes with a clip that is the perfect size to fit on the indicator lever. You want your mic as close to and as directly infront of your mouth as possible for people to hear you.
Even with the softtop down people seem to still be able to hear me with a bit of wind noise.
This is the one I have. Had it for 6 months now and love it
http://www.pioneer.com.au/au/product...0BT/index.html