Get a Parrot CK3100. No joke, totally awesome. Post which of the cars in your sig is hers and I'll link you to the easy-install harness for your car if it's available. I've installed one in my RAV4 and one in my wife's Highlander, super easy with the PnP Toyota harness.
Mutes the stereo and sound comes out the speakers. Processing box fits well behind the stereo, plenty of cabling to make everything fit where ever you want. Excellent audio processing removes minor to moderate wind noise and all echo.
Plus if your phone supports voice dial it will work through the Parrot kit. If it doesn't but it supports bluetooth phonebook transfer you can set up voice dial tags on the Parrot kit (!).
Can't. Recommend. Highly. Enough.
FWIW, don't spend the extra and get the color Parrot kit. They work the same for talking and the screens are MUCH harder to read with any glare. Not worth it for basically only getting a picture of the caller.