There's a couple FAQ documents on bluetooth variations linked to over in the mk5 forum. Sounds like you have the 9wZ module, which they don't provide much info for. But it sounds like a similar problem to swapping a 9w7 module into a car that originally came with the 9w2 or 9w3 module. The car is only wired for sound from the right channels. Calls are single channel, so they don't matter, but music is a different story. You'll probably have to add wiring for the left channel. Two wires from the bluetooth module under the seat up to the back of the radio.
VW Bluetooth module FAQ and how to swap or retrofit it in your Jetta, Golf, and sportwagen: 9w2, 9w3, and 9w7
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