Hi there and thank you for your question,
I will try to assist you with your legal question but please feel free to ask as many follow up questions as you like until you are 100% satisfied.
Unfortunately in my legal opinion you would NOT have grounds to demand that they change your radio so that it is the same as the demo model car's radio which you were shown. To me, that ground is not sufficient to return the vehicle. If (for example) the engine was the wrong size, or there was an important and material difference then you would have grounds - but not for a radio.
What you would be entitled to do however is to sue the company for a refund of the difference in value of your car with your radio, and the demo model car with its radio. As you can imagine, the difference in value is most probably very small - so it would not be worth it. Maybe R500?
Regarding the returning of the vehicle, again, I don't think that you have grounds to return the vehicle because the bluetooth radio isn't working properly. But you can demand that they fix it! You have rights in the Consumer Protection Act (and the vehicle's warranty) which requires them to fix the radio.
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Good luck and best regards,