To answer the original OPs question, I answered a very similar question a few weeks ago, whereby we came to the conclusion that you cannot always retrofit an aux connection to an RCD300 as it doesn't have the connectors (I still have a suspicion that the later silver button RCD300s probably can but not the earlier ones). The only other way is to retrofit the VW iPod prep kit or an aftermarket kit such as the products produced by connect2. Both these systems basically "fool" the RCD300 into thinking an external CD changer is fitted. This means you can choose playlists, etc by selecting different "CDs" but you will not get any track info etc on the screen with the iPod prep kit. I think some of the aftermarket kits do display track info but I'm not sure on this.
If you have a third generation head unit already retrofitted (RCD310, RCD510 or RNS510 most likely) then you can add the MDI kit which will provide full iPod integration.
Don't forget if you've got an RCD510 or RNS510 then the cheapest way of getting music onto your head unit is to put it all onto an SD card and stick it in the front of the unit!
