My reference to warranties on the Mk6 Bluetooth retrofit was not regarding the cars warranty, but the warranty on the retro fitted parts. You get a warranty on the genuine accessory if fitted by the dealer, but not on anything with third party kits.
As for wiring looms, years ago you had a one fits all body harness for a model of car. Obviously engine harnesses would differ. My old Mk2 Cavalier for example. I had a lowly model with no rev counter. Bought a set of SRI clocks from the breakers, plugged then in and they work. The same with front fog lights. The wires were tucked up behind the bumper. Plug in a new switch, plug in the relay and they worked too. When you come to retro fit parts into modern cars, no hardware is in place. Yes, the BCM is often ready and just needs coding, but wiring needs to be added, and 9 times out of 10 it's an aftermarket part.