You can now get the additional loom required to fit cruise control to a 1.8t (and other models) as a genuine VW spare.
For engines with the 80-pole ECU plug (Engine codes: APE, AXP & BCA)
1J0.971.425.B
For engines with the 121-pole ECU plug (Engine Codes: ARZ, ATN, AUM, AUQ, AUS, AZD, AZH, AZJ, BAD, BCB, BFQ)
1J0.971.425.C
The wiring goes as follows:
Engine Code: APE, AXP & BCA (80-pole plug)
Pin 24: Black/White Wire
Pin 25: Red/Yellow Wire
Pin 39: Blue/Grey Wire
Pin 50: White Wire
Engine Code: BAD (121-pole plug)
Pin 49: Blue/Grey
Pin 51: Red/Yellow
Pin 52: White
Pin 55: Black/White
Engine Code: BFQ (121-pole plug)
Pin 66: Red/Yellow
Pin 67: Blue/Grey
Pin 68: White
Pin 69: Black/White
Engine Code: ARZ, ATN, AUM, AUQ, AUS, AZD, AZH, AZJ & BCB (121-pole plug)
Pin 38: Black/White
Pin 57: Red/Yellow
Pin 75: Blue/Grey
Pin 76: White
Hope this helps