how do i download my phone book to my cars bluetooth?

I did exactly what you are trying to do with my Garmin Nuvi 650 GPS. I'll I did was link the bluetooth to the GPS and the phone book transfered without prompting. I have to believe that your car's bluetooth may be smarter than my external GPS so it may do the same. If not, you can always check you owners manual.

My iPhone would not download contact data to the BT handsfree system in my car - entries I'd downloaded to the car previously with my old phone remained and I was able to augment these by manual entry, but not from the iPhone.

What make and model car? It seems some manufacturers have implemented different BT solutions, some of which work and some don't.

My iPhone downloaded my contacts to my Mini Cooper automatically the first time I joined up with the Bluetooth (using a default password). Works nicely, and seems to add to the contacts in the car whenever it joins up.

I have a 2006 Lexus GS with BT phone, but I have been unable to upload the iphone phonebook into the car's phone book. I looked on the Lexus club discussion page and could not find anyone else who has been able to do it either. 😟
However, the iphone link with the car's hands-free is fabulous. Great sound quality, both outgoing and incoming:)