I haven't done the ones on my Treg, but on my other diesels (including a Golf TDI), you just pull the boot off and remove the plug with a deep socket. Kind of like changing a spark plug. The system will show a fault if the harness is damaged. Obviously, if you need a new harness, get it from the dealer. Plugs? Not sure if there are aftermarket plugs available. I bought plugs off eBay for the Golf (brand name).