DEAD 1972 plymouth valiant

Now you say, you "have nothing." What exactly does that mean? Do you have interior lights? Do the headlights/ tail lights work? Does ANYTHING work?

If the answer is no, refer to the diagram above. The main power distro comes power from the battery, to the starter relay 'big stud' which is a junction point. From there the fuse link feeds into the bulkhead connector (the big RED ammeter wire) to the ammeter and through it, out on the ammeter BLACK, and to the under-dash WELDED SPLICE. This splice branches off and feeds the HEADLIGHT SWITCH, the IGNITION SWITCH, the fuse panel HOT BUSS, and back out through the bulkhead connector (large BLACK) to the alternator.

Your troubles are likely........the fuse link you replaced is not making contact.........the bulkhead connector terminal(s) are bad, the ammeter terminals/ the ammeter is bad, the welded splice is AFU.

It also could be that "something failed" and is the reason the fuse link blew open. It might be that something in the alternator / field wiring/ ignition system/ other has caused that problem. IT MAY HAPPEN AGAIN

One thing you can do to prevent a short from causing further damage is to insert a large wattage bulb in series with the battery round. Use such as a stoplamp, or old hadlamp. The car will not crank, start, or run, but this will limit current flow and prevent "smoke"

Since you say the failure occurred when you turned on the lights, the FIRST thing I would check is the dimmer switch area. Since it is mounted to the floor, look for something broken/ bare wires/ a possible short in that area The headlights are NOT fused. They only have a breaker in the headlight switch, and it certainly is possible the main link would blow before the headlight switch breaker