Splice in at the brake light switch.

IIRC the Firebird uses the same filament for both brake and signal. To avoid the 3rd brake from flashing with one of the signals in that case, you need to connect to the brake wire between the switch and the steering column/turn signal switch.

You can use 3rd brake lights from other vehicles. I used an LED 3rd brake light from a Safari van on my Datsun. This is an external mount 3rd brake light, but it suits my car well.