Hi Pawnii,
The RCD-510 is the middle of the range audio system (Radio CD).
The RNS-510 is the top of the range navigation/audio system (Radio Navigation System).
The look slightly similar from the outside but are very different animals.
There is a handy comparison table here:
Differences between the Volkswagen RCD 210, RCD 310, RNS 310, RNS 315, RCD 510, RNS 510, RCD 550, RNS 810 and the RNS 850 | my-gti.com
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