You can pretty much grab power for your stereo from any power wire that you can find. Get a test light and find a wire that has power only when the ignition is on, that way you won't leave your stereo on by mistake and drain your battery.
If you are fussy about the sound or are putting in a killer unit, run a wire directly to the positive terminal on the battery and you'll avoid most of the engine noise that can come through the stereo. Remember to turn your stereo off if you wire it this way as it will always have juice running to it.
BTW, welcome the the forum my VW brother!