Awesome GTI tuning this week!
The Haldex controller may work if just plugged in, but without an earth it may malfunction and damage the circuitry. Apparently many of the controllers fail in this way, as earth straps often break. Your diff unit should have an earth strap and they are easy enough to make. There should be screw posts on both the shell and trans. See this picture of my Haldex unit which I'm selling.
Audi A4 B7 2004 red 2.0 TFSI Quattro Sport saloon, Servotronic steering, MFSW, CDIS, DRL with Coming Home, auto dual xenon lights/wipers/mirrors, split folding rear seats, 3rd headrest, centre armrest, Isofix mounts, RS4 rear ARB, pedals & carbon dash trims; Audi TTS 9Jx18alloys (2 sets), Yokohama AD08 summer, Conti TS830P winter 245/40/18 tyres, TPI 15/20 spacers/lockers, Pipercross filter, GFB DV+ turbo valve, NGK BKR7EIX plugs, Osram Silverstar Xenarc D1S bulbs. Build thread