I'm an employee of Volkswagen in Germany and one of the perks is that you can drive the cars. So far I have driven a Tiguan (with a RNS 510) and currently a Touran (with a RCD 510). The sound from the BT connection in the Tiguan was amazingly good: clear, pristine sound where you could place all the instruments in the mix very well. With that radio I didn't even need the cable connection to the iPhone. Than I got the Touran with the RCD 510.
Today I'm driving home from work when Fleetwood Mac's Everywhere comes on. The coda has a sample which should be heard on the left and right speakers of your car. This didn't happen and than I also started to question whether it is mono. Selected a song by Jean Michel Jarre, Oxygene 5, which is a perfect check. And sure enough, only one channel was heard. Mind you, this is the same iPhone which is connected to the RCD510 which, until 2 weeks earlier, was connected to the RNS510 and provided clear sound. Hence, the problem is not in the iPhone, but it is in the radio and/or the bluetooth configuration in the car.
Maybe bigbadvoodooguru's advice is good, but I would bring the car to the VW dealer to do the modification if I would be owning it. Or buy myself the RNS510, it's a much nicer radio with a much clearer screen anyway and you have an integrated navigation.
Just my 2 cents!