The whole VW wiring harness is a pretty complex piece of engineering. But when you break it down wire by wire it's not that bad. I wanted to keep everything needed to run the TDI plus retain things for A/C and fan control. I'm not sure if I'm going to keep the A/C, or which fan controller I'm going to use, but I wanted to keep my options open. I can always cut off more later.

I started the thinning by following every wire from the ECU to its end. When I got to the end I would determine what it was for using the Bentley manual and the termination type. Once it was labelled I could keep the plug if it was for a device I was going to use, or cut it off if it was going to to be spliced in to alternate vehicle system.

Below is a list of all the ECU wires and what their colours and function are. I'm not an expert on VW cars so I still have questions about what certain wires do. If you have any insight into these please speak up.