There's been a bunch of posts on this.
There are basically three ways that I know of:
1) Tap into a fuse
2) Tap into a connector below the dash - what I did
3) Tap into the dome light

I know all of these methods have been described here
before with pics and instructions. I keep meaning to
gather all this stuff together and put it somewhere,
but haven't yet. If I can find a link, I'll post it
here.

scot

Scot's TT Page
'01 TTQC, '02 S4, '56 Bug (ex, '66 Ghia)
TT Mods:
Bilstein HDs, Optimus DV, APR 91/93,
Schwing TTDA, Forge FMIC, Neuspeed
Rear Bar, EFU (boost/oil temp/voltage),
Blue Flame Exhaust,
Winter Mats, Stoneguard, V1,
TT Stem Caps, Rear mudguards,
Zaino, License Plate Vault, Raffi Badge,
VAG-COM, Bentley Manual, Ipod aux-in
Summer: Goodyear Eagle F1-GSD3s
Winter: Pirelli Winter 210 Asym Snows

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-Scot aka ZZCETT