Well this took a little bit of work, this is how to install a 3rd brake light in a monte that is 85 and older that did not come with the 3rd brake light, and no you cant just splice into the taillight wires on the back of the car, your third brake light will blink with the turn signals.
First things first, you cannot just use an 86 and newer taillight harness, it does have the light blue wire for the third brake light but its recieving plug has no pin for the light blue 3rd brake light wire so....
here is a picture of the 86 and newer harness, notice the light blue wire in about the center of the plug, on 85 and older harnesses it does not have this wire
heres a picture of an 86 and newer fuse box where the taillight harness plugs into, notice the 6th pin from the bottom, it is gold and this is the reciever pin for the light blue wire on the taillight harness, 85 and older will not have this
the reciever pin on the fuse box has a light blue wire going to it from the brake light switch on the pedal, but the 85 and prior switch is different then the 86-88 switch, the 85 and prior only has an orange wire and white wire, the 86-88 has an orange wire and white wire and also the light blue wire that goes to the taillight harness plug on the fusebox
notice the difference in the switches, only difference is the added blue wire and also the plugs are different so you will need to get the switch and plugs off of a donor car
86-88 with third brake light is on the left, 85 and prior on the right
so now the plugs are different, heres what i did, i took the plug off of the newer brake light switch and added male spade connectors to plug into the factory plug in the car
i then plugged them in to there respected sides matching up the colors
-now for the orange and white wires, i just cut off the factory plug on the car and butt spliced them to the new plug
-now you have the light blue wire left to connect, just connect a long piece of wire to the light blue and run it to the + on your third brake light and also ground - wire on your third brake light
-next install the new brake switch back on the pedal bracket and adjust it so the brake lights are off until you push the brake pedal
-the last thing i forgot to mention was that to install the 3rd brake light you will need to cut a hole in your rear package tray for the braket, before i removed my package tray i just went through the trunk and marked the back side of the tray through the existing holes in the rear deck, so then cut the hole in it, bolt on your bracket, install the package tray and then install the 3rd brake light
---------------new third brake light!!!!