best way to bypass rear break light.????????????????

i have a probblem and carn't seem to find the cause would it be easy to
make a 12v bypass from one on the 12v batteries ? it has 4 12v batteries
break switches are working fine how would i go about wiring the switches to the rear light
so the rear break light works as the swiches are getting a 12v suppliy to them but somehow its
getting lost throught the electrics and is showing from the three wires to the rear light
one is green/yellow .( middle wire is green ) and the third wire is brown ......
well . 1.... when i connect the volt meter up red+ probe to the on the green and black - probe to the brown wire reading shows 2.64v = rear side light
2..... when i connect red probe to middle wire green and connect black probe up to yellow/greem wire reading is =2.80v
3....when i turn off the side lights volt metor shows 3.70v thats with red+ probe to the brown and black- to green/yellow but if i turn on the side light on the metor shows 00.00
4.... am confused as to what to do next ..
all comments appreciated. kev