Here's a snip of my schematic for A3 wiring showing just the cruise control switch and the connections needed.
View attachment 128088
Power from fuse 11 black with blue stripe, to black, to white, to cruise switch white.
Switch blue, to blue with grey stripe, to ECU pin 34.
Switch red to red with yellow stripe, to red, to ECU pin 35.
Switch black to black with white stripe to ECU pin 66.
My A3 schematic doesn't describe the locations where the connection points are, but my B4 schematic, nearly identical image, same connector IDs, fuse and relay block power source, has T4a in the column behind the steering wheel (once the switch is installed, that is), T4b as a four pin connector point behind the fuse and relay panel (probably the most difficult part of the option install to access), and TV5 above the fuse and relay block.
My experience with that era's VW wiring was that the harnesses were fully populated, meaning that, even if the car didn't have the option installed, the wiring at both ends was present, it was necessary to only plug in the missing pieces.
That was true for adding optional fog lamps, adding a CD changer, power seats, seat heaters, and so I expect that the cruise will just require installing the switch, connecting it to the four wire 'extension wire set' at pin-socket T4a, plugging in the T4b end of the 'extension wires set' to an existing mating T4b behind the fuse block, and connecting to the power at the "TV5" connector already present.
Upon review, I expect the TV5 connection to T4b is already present as well.