The third brake light basically gets its +12 volts from the same area as the normal brake lights.
The earthing points are different though.
First thing is to check for +12 volts at the rear bulbs themselves...with reference to the earth on the sockets and then with reference to a good, clean chassis point.
It is quite common on these KJs for the sockets themselves to get dirty/burnt and it is also common for the grounding/earth point to get lost. In the latter case you can make up your own earth by drilling a small hole into the bodywork inside the rear cluster assembly, clean off the paint and screw down your new earth lead using self-tapping screws.
