Check the brake light fuse first and if good you need to replace the brake light switch that is on top of the brake pedal under the dash.
Brake lights should NOT be effected by the headlights. Brake light function is completely independent of the headlights. Assuming you are not refering to the tail lights, or marker lights, which are supposed to come when the head lights are turned on. Not having brake lights when the head lights are on is a big safety problem and should be corrected.
The position lights on my XC70 are the bulbs just above the taillights/brakelights on the rear of the car. I have one that is often going out and generating the position light message. I usually just rap the brakelight area with my fist and the light pops back on. I guess there is a loose connection in there.
you need to get a bigger fuse
Check grounds in light sockets Check for corrosion and loose connections
Yes, a 1979 Firebird tail light will fit a 1979 Trans Am. There are also restoration services if you have the tail lights and they just need to be repaired.
Check the brake light switch on the pedal. There are different switches for the 3rd and reg brake lights If you have already replaced the bulbs of the two lights that are not working then check the wiring between the lights. If you don't have a repair manual get a haynes it has wiring diagrams in them.
Check the operation and alignment of the stop light switch
the 1973 f body fuses are separate for park lamps and dash lamps, so it must be in the light switch itself.
You need a minimum of 2 brake lights, So if your car has 3 brake lights and one goes out, you are still legal. Some cars do not come equipped with 3 brake lights but others do such as a honda civic with a spoiler light do.
put a spoiler on it
You probably won't have dash lights either. The problem will probably be at the light switch. If you take out the switch, there will be a burnt wire, and the switch will have to be replaced, along with a pigtail for the burnt wire.
unscrew the light from the spoiler Disconnect the water hose Viola