Im in a similar situation at the moment too
Ive fitted a RNS510 in to my mk5, Ive been looking into this for a few weeks as the last thing I want to do is blow the new unit up, so the answer is to buy a High level adaptor/convertor which as rightly said above u connect the unit to your speakers, remote wire can be picked from the climate control unit too.
The most important thing to me is that you must buy an AMP that has an adjustable bass/sub level control as the RNS510 does not have them controls.
Hope that helps mate