It has been so long ago I would have to go by a manual to know for sure. Keep in mind my knowledge is general knowledge from many makes and models spanning many years and my memory is not what it used to be.
I remember older fords have fuseable links at the starter solenoid mounted on the fenders they look just like a regular wire except the insulation is normally stretchable a simple tug on them normally will show you it is burnt because it will stretch with no resistance and sometimes they will just be burnt in two.
I am not 100 percent sure that the 1979 TA has fuseable links. GM often would use circuit breakers also that look like little metal relays found in the fuse panel.
So lets check the basics,
Do you have a 12 volt test light or a DVOM ? if so start by testing each fuse in the fuse box use the light to make sure you have power on both sides of each fuse with the key on engine off. You have the old style glass fuses still right?
any fuses that do not have power on them with the key on engine off check with the engine running and with the headlights on.
if you find any fuses that do not have power at all under these conditions write down what the fuse is labeled for. if the markings are no longer visible
it will make diagnosing the problem more difficult.
If you have anything on the fuse panel that looks like a metal relay those will be the circuit breakers one may be labeled for the headlights. again I am not sure, these breakers were used for power seats for sure but I do not remember if they were used in the lighting. for now we are looking for what has power and what doesn't.
to test these circuit breakers provided you have them and you can still see the label identifying the circuit you may need to pull them out one at a time and make mini jumper wires so it can be connected while you have enough room to probe the electrical connection to check for power on each side of the breaker they are just like a fuse that will reset itself but the body of the breaker is normally is too big to probe the connector while the breaker is in place.
GM and other manufactures normally sent the main power from the battery directly to the starter solenoid and a smaller one to the alternator. these fuseable links if they exist would attach to the solenoid or the alternator the fuseable links would feed the main power to the fuse box. this way no wire penetrating the firewall directly from the battery would be without protection
My memory is starting to come back Gm had a small metal sleeve near the starter that some small wires would go through this metal tube protects the wires coming from the starter. in that location if you have fusesable links that is where they would likely be. So if you checked your fuse panel and any of the fuses are not powering up disconnect the battery and look near the starter for a metal tube or bunch of wires.
you may have to disconnect these wires to get a good look at them
they can be easily checked with a DVOM without connecting that battery by checking for conductivity from the connector and probing just beyond the factory splice normally about 12 inches or so the fuseable link is just that a small section of wire spliced onto a normal wire thus protecting that circuit
If you have headers and no metal tube you may have a good chance to find your problem just because of a burnt wire which should be easy to spot.
Here is a link to another forum that shows the fuesable links are by the starter so I am not completely forgetful
http://www.78ta.com/HTAF/index.php?topic=14077.0
Good luck remember disconnect the battery before trying to fix them
