This is also posted in the MKV forum. I decided to make this post because I asked for a few tips on installing a sub in my GLI but for some reason, I got no help. Being that the tex has always helped me out before I figure this is because not many people know the MK5 very well yet. Well, I decided to take it on and make this post. As everybody else says, I'M NOT RESPONSIBLE IF YOU BLOW YOURSELF, YOUR CAR, YOUR KIDS OR YOUR WIFE UP.
EDITED: New info on remote connection
Overall, It was pretty simple but took way more time than usual (about 6 or 8 hours over 2 days). The MKV is put together very well, I actually managed to not break any trim peices or scratch anything up. This post is assuming you are using the factory head unit but the actual wiring may change for cars with different options. (see below)
Tools and Items needed:
Sub and amp (obviousely)
Correct amp wiring (kit)
line out converter
STRONG drill and lotsa bits
Torx bit/driver (either T20 or T25 i forget)
ratchet, long extension and 13mm socket
Multimeter or 12v test light
little metal doorsill clips(dealer? see below)
First off, you will need these clips, They are pretty much one-time use and will be garbage when you pull the doorsills up. I am guessing you can get them from the dealer. If you cant fint them right away, the sills do stay on without them but you are better off having them.
First you have to remove the battery and box cuz it's tight and you will need the space. remove the top and front of the battery box, unhook the battery cables with the 10mm wrench then use the ratchet, long extension and 13mm socket to unbolt the clamp on the engine side of the battery, then undo the back of the box. The buttom does not need to be unbolted.
Now you need to look for a hole in the firewall. I believe DSG/automatic cars will have an open hole or 2 where the clutch pedal would go. Unfortunately I couldn't find one in my manual GLI so I had to drill. If you have to drill, you must be very careful, the firewall is very cramped in the engine bay side so you must make a mental referance of where everything is in there cuz you have to drill from the inside. Drill 1 large hole for the power wire big enuff to fit a grommet in. EDIT: The 6disk GLI stereo has NO REMOTE, I confirmed this myself by checking all wires in the head and even the prongs on the stereo thatt do not have wires in the harness.
Now you have to pull the driver side door sill up. first insert a flathead screwdriver here
Then lift it up just enuff to get your fingers in from both sides, then pull it up. All those clips stuck in the metal under the doorsill have to come out and be replaced. The whole sill is one piece from front to back so just follow it down to the back seat. You will find a small styrafoam piece in the front of the doorsill, either toss it or drill a hole through it to stick the wires. The piece where the hoodlatch release handle is held does NOT need to come out. you can just stick the wires up through the bottum
The bottom of the backseat has to come out a little as well, it is only velcroed and snapped in. Pull it up from the front until it pops out. It's hard to get all the way out from under the seat belts so popping it out a little will do.
route the power and remote wires through the plastic piece with the hoodlatch release handle, run the power through the fire wall. Then run it to the back under the seat, then through the trunk. Put new clips in the sill and pop it back on. Put the battery back in but do not connect amp power yet. You can now stop for the day or keep going. EDIT: run the remote wire up through fuse panel on the left side of the dash and to one of the spare 10 amp fuses in the middle right side of the box. these fuses are not used for anything in the car but do have 12v running to them for the ignition.
Now you have to remove the head unit. I didnt take pictures of this but it's all detailed here. http://forums.vwvortex.com/zerothread?id=2111976 Just use the butter knife to pry the trim off then use the torx to unscrew it all. Wiring info is in the link and on back of the deck. You need to splice into the rear speaker wires. Stuff the speaker wires down behind the climate controls so you can pull them later.
Pry off this piece under the climate controls from the front then remove it.
now pry this piece off from the pass side of the dash
You have to remove the glove box. Unscrew the torx screws from the inside and underneath. unclip the wires as you pull it off. If you didn't notice there's 1 screw for the glove box under the climate controls.
Now pry the kickpanel and doorsill up. Pull the wires from under the radio through here then over the glovebox and down through the door sillinto the trunk.
Hook up your speaker wires to the line out converter. You can use the 10mm nut under the seat or the large triple square bolt if you have the tool. Hook up the amp, sub, whatever, then connect the power to the battery. Oh, you have to drill a hole in the front battery box and run the power wire in it. Put it all back together, check all connections and you are all set.