I need to conect my Karma to the headunit in a VW Touran, there's no appropriate CD Changer to AUX/RCA cable, I assume that this means my only option is an FM modulator, but are they any good?

I have an Audiovox FM Modulator that I use with my Empeg. I get no interference, because it connects through the antenna jack on my stock HU of my car (I also cannot get a CD changer aux-in interface for my Subaru Outback). It sounds like the best radio station you have ever heard, basically. If you can live with that, then you'll be ok.

Here is a pic of my setup. I use a relay connected to the Empeg's amp sense line to turn on the FM mod, you'll have to hook up a hidden switch somewhere under the dash/glove compartment if you are using it with a portable.

Thanks for that Mark, I'm looking at one by Veba which seems very similar to the Audiovox.

I've listened to a lot of brands of FM-Modulators, and they all pretty much sound satisfactory. The only thing I would suggest is get one that hooks into the antenna (as mentioned above) and not a wireless one. I have yet to hear a wireless one that is close to satisfactory. I would expect to pay around $40. You could find one cheaper, but you get what you pay for.