Ok in my Monte (81) the white color cable traces from the brake switch all the way to trunk.
This is the wire that must be tapped into as your hot wire. It only comes on when brakes are pressed (of coarse)

If you get stock Monte with 2 prong. Its best you get the harness plug. Keep the plug end that goes into light and cut the end of plug that is suppose to go to harness on left trunk side that goes up to fusebox and taillights.
I tapped into the line right before it goes into pin. Its a THICK wire, not many little strands.
Blue goes taps into to white
Black goes to any ground

Same applies to aftermarket third brake light.
Power wire taps goes to white
ground wire goes to any ground

A Little Background:
Previous owner had a aftermarket looking 3rd brake light installed in mines and I wondered why it never "worked"
Well it was the way it was being tapped in
It uses a Logic Box...2 wires to got to left and right tail light and 1 power that goes to the light hot wire.
The way mines is...at the harness only 1 wire is constantly hot and the other is (the white we tap into that goes to brake switch)
You would have to tap into DIRECTLY behind tail light (between the tail light and harness)

So there is NO NEED TO USE A LOGIC BOX.
Just tap into brake switch wire, every things already ran for you.

 

Sigh, all this is wrong.....I had wiring issues........if your car was not made for 3rd brake light, logic box is still needed...and tap it into left/right brake light!

Also, Cereliannc, what are you talking about?

 

He is a spam bit, just reported it to a mod should be gone soon

 

Taken care of