If you are getting your stuff from Crutchfield, they will include some great documentation that will help you properly install things like speakers and the dash unit.

I recommend using the factory speaker wires, harnesses, and connectors wherever possible because that will be the easiest and least invasive to install. Again, if you get your stuff from Crutchfield, you will get these adapter plugs for free.

Do not clip off the power antenna wire, Volkswagens need the antenna amplifiers to be powered. You might need a special adapter for this depending on the model of VW.