I want to put an aftermarket brake light (3rd light) on my nissan 1400 bakkie. I know the wires must join the existing circuit, but I cant find/access the excisting brake light wires. where are they more likely to be?
Any help would be appreciated.

Well, I guess you have a choice of: -
1. at the rear tailights/brakelights, probably not very helpful since I'm guessing that the 3rd light will be in the cabin, so,
2. at the brake switch which is a plunger switch on the brake pedal.
Usually the plunger switch applies power to the lamps, so you will need to earth the other side of your 3rd lamp.

Wires will be down the drivers side floor, probably like the sedans/coupes, it will be a flat multiconductor wire ribbon You can cut open the insulation on the wire, once you figure out which wire it is. Use a multimeter while pressing the brake pedal, to find out which wire it is..

thanks guys ill try some more.
if i still have difficulty , well u guessed Ill be back.