One of the Parott systems. If the existing in-car HiFi system has DIN connectors (ir probably does), fitting is a doddle. If not, adaptor "Y" leads are available for most vehicles.

I fitted a Parott 3200LS to the wife's Zafira. Box 'O tricks ziplocked to a mount underneath the glovebox and tapped into the stereo connectors with an adaptor lead. Main lead through dash to head unit, mike run up behind passenger door seal and behind headlining to mirror. The end result feels way more "integrated" than the built-in products in many cars.

Total cost was a shade over 100 quid including the third-party adaptor lead, took about 2 hours to fit. I have yet to find a phone that it wouldn't work with, although some do not support the phone battery charge indicator on-screen. It will automagically use whichever of its paired phones are in the car at the time, with a default one to use when more than one is present. Phonebook is downloaded from the attached device at connection time, so updates are automatic and you get the contacts associated with the phone rather than what's stored on the car. Both of these features are essential when more than one person uses the car and most integrated systems FAIL here (including Toyota's - if you can get theirs retrofitted, don't bother).