The best place for your Xovers is in the boot really, especially if you are running them from an amp.
Short of that, in the doors or under the dash or glovebox ( as said above) if you don't mind loosing it.
My tweeters are in the mirror backs, took me only a few minutes and cost a total of 0 pounds and 0 pence to do. I may be able to get a photo of them at some point but you would not even know they were there.
I will be doing some A pillar builds soon when I get my a** into gear so maybe we could sort out a swap, my mirror backs for yours!

mine is not a GTI and is a convertible so they would probably be different TBH.
You don't really want the tweeters in the standard position in the dash, it is far from an ideal place.
Neither do you want components in the rear of the car, its a pure waste of money.