Abs light on and ESP light

Hi I have had a abs fault on my car for a bit now I have changed abs sensor and had abs module tested and now to try and find out more I have tested the green cable from abs sensor to abs module plug but does the black cable go back there as well I only have 6 volts on it is this right and how do u change brake light switch as I can only get finger tips to it any one help please I have more faults to clear but think this one is more to sort out first cheers andy