I just installed a sub in my MkV R32.... Firstly, does your sub have a built in Hi/Lo converter?
To run the power you can fit it through the hole in the firewall where all of the other electronics come through. There are plenty of write ups and DIYs on the net if you search for them. It is best to run it along the left side kicker panel and then up through the left side moudling near the back seat. Just make sure there is plenty of spare cable as it is easier to make it too long then trim rather than too short to begin with.
If your amp has the hi/lo converters you won't need to have an aftermarket ho/lo converter and can simple tap into the speaker wires coming from the back of the HU and run them to the amp. For the remote turn on you can tap into the 12V socket in the boot of the car. You want to tap into the red wire with the black stripe which is the negative.
Ensure that you keep the battery disconnected through all of this though as I didn't and blew my 12V accessories fuse and spent ages trying to figure out what was wrong.
Also remember to make your grounding wire as short as possible... mine was intially long and was causing the amp to turn on and off randomly. The shorter the better is what they say!
2008 Mk5 Golf .:R32/ BMP/ RCD-510/ More to come