I have an 85 z28 without the spoiler mounted third brake light, but i want to add one. i already have a spoiler with the light instaled on the car, just wondering how others that have done this hooked up the light.

This is how i would do it.

Get a cheap $10 voltmeter and a buddy.
Take of one of the taillights off. Take one of the bulbs out. With the meter touch the neg(black) lead to ground or any metal on the car. Have your buddy push in on the brakes. With the meter, take the red positive lead and place it on one of the contacts that the bulb was in. (Make sure you dont touch two contacts at the same time, might cause a fuse to blow.) When you find the contact that has 12 volts appearing on it, then that is your brakelight wire. Splice into that wire and connect it to one of the wires for your 3rd brakelight. Take the other wire for your 3rd brakelight and connect it to any metal on the car, this will be the ground wire.