Finding the 12v Feed to the rear light
So turn on the ignition (DO NOT START THE ENGINE)
Ensure the car doesn't roll if your on a slope
Engage the reverse gear only (Ensure all the other sidelights/hazards/headlights are off)
With your multimeter - Set to 20V DC
Locate the Black wire - this is the earthing pin
Using the positive probe locate your reverse light wire
They all should read 0v until you hit the reverse light and you should have a 12v (ish) feed.
If you have more then 1 pin reading 12v, double check all the lights are off and try again
NOTE - be-careful as you probe the pins, i managed to short the reverse light pin and earth pin with the probe, resulting in the fuse blowing
So make a note of which wire is your 12v feed, this is what i had