Not 100% clear on what your question(s) are but will attempt to answer..
Yes the RNS510 uses an external module which is located under the seat in the Golf etc from the factory.
If you want to know if the Touran has a bluetooth module already, try pressing the phone button on the steering wheel. If it just mutes the radio and doesn't give the phone menu in the MFD, then it does not have a BT module.
Alternatively you can do an autoscan with VCDS to see if there is a module at address 77 (telephone).
There are aftermarket units such as Kufatec etc. If a BT unit was retrofitted it could have been placed under the head unit, or somewhere else inside the dash.
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