Vw, actually sold a number of kits; that were oem ipod interface adapters... there are allot of aftermarket solutions as well. If you wanted the VW adapter kit; it usually took out your ability to use the disc changer... which doesn't seems to be an issue with your situation. The VW adapter kit; made it, so you could control the ipod through the stock head unit. This was a good solution; if you didn't need to use the disc changer, now... all these years later, which ipod it would now to compatible with etc. I don't know. I"m assuming it would have been the current ipod at the time; so "forward compatibility" is questionable; you would hope the tight apple/vw integration would increase the possibility they would still work. Maybe others; can chime in with their beetles that have this older vw adapter kits installed; can tell us about forward compatibility with current apple/ipod/iphone units that work with these older setups.
So, they seem to have come down to:
A. do you want to use the factory head unit?
If "yes"; then the solutions were:
1. an adapter that was more of a "aux out" type of solution without head unit control of the ipod. You controlled the ipod with the ipods built in physical buttons; basically played through the head unit without much controlling integration.
2. an adapter that used the disc changer connection to actually control the ipod through that connection, eliminating your use of that, for an actual physical disc changer.
There were a number of the VW ones they made; so, research would be needed to see which one was compatible with your beetle and head unit. Here are some listed from ECS tuning.
Home Page > Search > 1K0051444 - ECS Tuning Inc
So, the question would be; what you want your solution to do for you ultimately and then see what your options are. Do you want full integration with your head unit (control ipod through the head unit) or just want a "aux in" type of adapter or as jfoj mentioned, something that had more modern hands free connectivity compatibility (wireless, bluetooth etc.)?