Well the thing is that Volkswagen runs their data thru the cars radio, when I installed mine I had to options which I will list below...
1. Hard wiring - this is the cheapest way to go, however it will throw an airbag code and your CEL will stay on. Also if or when you have to go get emissions done, assuming you live in an emissions state, then you would have to uninstall the aftermarket deck and reinstall the factory radio in order to clear your CEL codes to be able to pass emissions. Also if you ever have any REAL issues with the car in anyway that throws a fault code, you won't be able to connect a reader and test for any codes.
2. Bypass Module - this is an expensive option however it is the best way I recommend installing ANY aftermarket head unit. Installing the radio this way will ensure your car does NOT throw an airbag or CEL. And as you can imagine if you ever need to connect a code reader to your car there won't be any problems doing so. And emissions wouldn't be a problem.
The bypass module to do this install cost me $90... From a reliable audio installer. As a matter of fact he is the one that convinced me to go with the module and thoroughly explained why. I trust his judgement as he has been a good friend for years plus a professional installer for the past 12 years.
Also, to answer your question about being able to still maintain use of the steering wheel controls. It is possible, there is a separate module for that as well. I believe it costs around $100 from a shop but he directed me to ebay where the same module can be found for $50 its just a generic module. Think I'd rather do it right and spend the extra $50 for the quality module. Anyways, I hope that helps answer your questions.
