I have never known of a handsfree kit to be of use in a ragtop, I have had the hard wired type along with bluetooth enabled devices. Either you can not hear them well or all they hear is background noise.
But you can always try for yourself, if multiple hookup's are an issue for you, then combind the devices.
They make some nice GPS/Bluetooth mobile phone devices. Just pair your BT enabled phone with the GPS and make and take calls using the speaker in the GPS.
Garmin has a few like
http://www.garmin.com/bluetooth/compat-gps.jsp
-cheers
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