As for the plumbing... very simple and took a little over an hour
Just drill a small hole (big enough to allow the passage of the live and earth wires for the bulb and the vacuum hose) at the back of the moulding within the dash on the driver's side. All three vents are housed within a frame and this can be pulled from the dash completely. You'll see what I mean once you've removed it.
This site proved useful. Note you do not need an inline fuel filter to reduce buzzing etc. Mine's as quiet as a mouse without.
http://www.drwombat.com/audi/awe/gauge.html
You need to take the lid off the ecu water proof box and the 5th screw is a pain in the ar5e to get to. Once off you can see straight into the driver's footwell. Route the hose into the ecu box and drill a hole for it to pass through into the scuttle panel. I used some silicone sealant around the hose and hole to be sure no water can get in.
It's a pretty straight forward install. There was a post on the audiworld forums about this, but the guy tapped into the wires for the iluminated flap adjuster (ooo err lol) to light the gauge. I thought the wires looked too thin and delicate so hooked it up to the fuse box.
I'll try to find his post again.
Jez