clamp the croc clip to a clean earth and use the point to stab each wire on the light plug to assertain which is which ( you will need a mate to trace the brake light wire
sorted it dead easy thank god just as well had to tow a tipo 1.9td on saturday all the way back from high wycombe 450 mile round trip in a 170,000 mile marea td and no hicups at all,best bit it only used 30 ltrs of fuel there and back ,bl##dy great