ALSO i have a few pieces of advice for anyone setting up a new system in a mkv 2 door.
one, the speakers in the front are three ways. one is 6.5 inches and will need an adapter for depth, one is 4 inches and the other is small tweeter (0.75 inch or 1 inch, not sure, but doesn't matter if it's to be replaced).
two, most steering wheel control adapters won't work for this car. i installed a metra setup (9003-NAV and ASWC-1) to my gti and only 3 buttons actually work: volume up, volume down and mute. this limited functionality is common to a few different brands but it appears as though the PAC AUDIO part will work. i've been in contact with their techs (and metra techs) and from talking to them it looks like there is full functionality on all 8 buttons. that being said i haven't done it myself because spending another $250 for a few more functions is unnecessary for me. bottom line: DON'T buy metra for steering wheel control. PAC is the only option for full functionality.
three is for anyone looking for an antenna adapter. i bought the eu55 from metra only to find out that my car has two antennae and two leads. the metra adapter you need is the eu56. it has two leads put together with a little piece of plastic. get the plastic off with a screwdriver, connect, then tape it up for mechanical security. i've read a few things on here saying that the OEM stock head unit amalgamates the two signals, but (as an electrician) i don't think that's true. a signal is a signal. the more surface area the better. my signal on AM and FM with the eu56 has been perfect.
four. hook up the antenna remote lead to the red wire. i originally spliced the blue from my head unit, the remote lead from the sub, and the blue from the antenna lead all together. this MAY work depending on the vehicle BUT has downsides. the remote wire from the head unit has limited amperage and may not be enough to power multiple devices. to ensure everything works all the time what i did is connect the antenna remote lead to the accessory power. this will give the lead the 12V it needs while ensuring it has all the amperage it needs. (to get technical, the remote and acc 12V wires put out the same voltage. the only difference it the 12V has a much higher amperage limit. the bottom line is that remote wire and 12V wires do the same thing)
five. this may be obvious for some. it's for if you're putting in an amp. i put my amp under the passenger side seat. when installing the ground, be sure to scratch away the paint of the surface you're going to ground to. for example, my grounding (negative) point was the chassis between the floor and the seat bracket. i had to scratch away the paint on the body panel and on the seat bracket to ensure a solid ground. most paints are ungreat conductors so scratching away paint can do a world of good for good health of your components. yes, it may cause premature rusting but that would only come through negligence of your car (sealers are available for post-install as well)