Iphone TomTom car kit

I am using a 3GS with OS 4.0.1. I have bought, and returned, three TomTom iphone car kits because of an error message and the inability to pair with bluetooth. On connecting the phone to the unit and plugging in to the car charger, I get a message about this accessory not being optimised for this iphone and continues about reduced sound and poor gps reception. I opened the TomTom car tool app and it says no fix and that's it. The bluetooth just says searching.
I think this message is the bit that screws it up although on the blurb for the TT tools app, it says it disables an error message on first connection but never says what the message is. On mine it does not.
I do not want to roll back to 3.1.3 to see what happens then, I am quite happy with OS 4.
Anyone have any ideas of what I can do please? I have managed to get my money back from all these but still want to have one. I did try a Mio car kit and it paired but got no sound out of it whatsoever.
Thanks
Doug