I thought it might be helpful to have my solution to a brake pad warning light problem on the web in case anyone else searches for it in the future. My car is a 2008 A4 B8.

I had a problem where the brake pad warning light was always on even though the pads were new. The wires that led to the brake sensor in the latter part of the harness were corroded (totally black!) and so when I chopped off the sensor, twisting the wires together wouldn't work. I either had the option of replacing the wiring harness or finding another solution. The expense of the new harness and the faff taking off wheel arch covers and pulling the cable through, etc made me decide to do the latter. I didn't know where the cable led to inside the car so I jacked the car up so that the wheel dropped (could have removed the wheel but found that unnecessary) and then I followed the cable up to the top of the wheel arch and unclipped the two clips that were holding the cable up (pushed the clips through from the engine bay side). This released the cable which at this stage has four wires inside - two for the brake pad sensor and two for the ABS. I carefully scratched off the plastic sheath of the cable to reveal these wires. I left the yellow and red untouched (they are the ABS wires) and snipped the brown and black wires then joined together the brown and black wires together. I decided to solder but a clip would be fine. I then taped up the cable and fastened it to the engine bay again.

Hope this helps someone else in the future as I couldn't find another solution on the web.