My parent's company used to sell this stuff. I still have a ton of units (does anyone want them for free?) that do exactly what you want but they are designed for the older Nokia units that plug into the base connector. We bailed on all the projects before we got to the 2.5mm jack phones and Bluetooth a few years ago.
I would reccomend getting a unit that has a microphone that attaches to your lapel since we had a lot of mic problems when used in the car. Interestingly enough we drove around in my mom's 300SDL to do testing
In terms of specific models out there the RF units on the market were all pretty bad in terms of output audio through the radio. So try to find a place that doesn't charge you a restocking fee if at all possible. We gout our to work very well but weren't able to build them for all of the models of phones that we would have liked to due to engineering limitations.
Good luck!
P.S. I also have a ton of little capsule sized FM autoscanning radios that have a handsfree wired mic and earbud that plug into 2.5mm phones? Would anyone want some of these things?