the ecu earth points (2 of them) on the plunum are located if you are at the front of the car with the bonnet up.... at the back right hand side of the engine on the lower back torx screw in the inlet plenum (there are about 10 around the edge of it holding it in place. these two connections can become corroded and the torx screw can become loose, i ened up recrimping mine and then additionally, running a big new earth lead from there to the gearbox/starter motor earth bolt.