The car has armrest/iPod prep and RNS510. It's a used car from a dealer but should I still be insisting that they provide the missing iPod connector cable? And is it worth having? As far as I can work out I have the option to load all my MP3s to the HDD so I never have to plug the iPod itself in at all. Is that you all you guys are doing or is there a compelling reason for me to demand a cable?
Yes, I know this is a popular topic and I tried searches but no exact hits.
A car with the ipod prep has no cable to attach the ipod. You just dock your ipod on the connector and you should then be able to play your ipod via the head unit.
If you want to play your sounds from a hdd then you'll want something like the PhatNoise http://www.phatnoise.com/vw/
hth
I have just changed my car, now a BMW owner

I have a Phatnoise for sale if your interested. PM with any questions. You will need the cable from the Stereo to the Phatnoise, it has an 80gig Hard Drive and can be controlled via the stereo.