After I tackled the antenna installation, I had to get the radio installed. I've seen several different ways of approaching the issue, but I've never seen one I was really happy with. Here's what I did for a seamless install.
Browsing around on a Mk V Golf forum I ran across a cubby that European models have that is not included in the US model, due to some additional bracing behind the dash. After looking at measurements and photos, I finally decided to attempt this and ordered the cubby.
The dash and fuse box limit how wide you can go, and the radio I chose, the Kenwood TM-D710, is about the largest that will work. This radio has a main unit that contains the antenna and mic ports, and a remote head unit for control.
To keep cable runs convenient, and since the speaker is also in there, I decided to mount the main unit under the driver's seat. If I didn't have the factory stereo amp there, I'd mount it to the floor, but instead I had to attach it to the seat frame. There are some under-seat drawers you can get that already have mounting points in the seat bracket. On the Mk 6 forum, I found a post showing the mounting locations. I used the two central holes labelled Torx-20 and tapped them for M4 screws like the drawers use.
Of course the radio mounting bracket needs to sit 90 degrees from those holes, so I used a piece of aluminum to make a mounting plate. I had to make a few cutouts to go around the connectors attached to the seat bottom. You can also see in that image that the bracketry where those screw holes are located is recessed from the rest of the seat, so I also had to make some spacers to support the adapter.
To mount the head unit, I cut two pieces out of the sides of the cubby drawer to allow the radio to be recessed flush. I also had to notch out both sides of the trim piece to allow the cubby to close. Because the drawer face is so much wider than the opening, and you are at a pretty steep angle, the cutouts aren't really even noticeable.
I used the factory dash mount to actually mount the radio into the cubby. The mount is pretty flexible, but isn't quite tall enough to reach to the bottom of the cubby, so I used a few more small pieces of aluminum to extend the bracket.
You do have to cut the stops away that normally keep the cubby open to a reasonable angle, so I had to rig up a piece of wire to keep the drawer from just flopping down when you open it. This worked ok for a while, but eventually fatigued and snapped, so I'm still working on a way to remedy that.
Then, just mount the cubby and run the wiring using your favorite DIY guides, et voilà! You can't even tell it's anything special when it's closed!
The only thing I don't like about this setup is that the cables come out of the left end of the head unit. They would be much for convenient coming out of the back panel. I actually had to modify the 8P8C connector a little to get my wiring to clear. In the future, I am going to look at relocating the microphone port to the center console and routing the audio through the car stereo.