Stuart - Yes, in some instances the resistance can decrease. In silicon resistors, the resistance will go up, but in certain types of wire (tungsten) the increase in heat will actually decrease the resistance somewhat. Since there are no headlight relays according to the digram Tiny sent me, that isn't the problem. You also may be right about the ammeter being hooked up backwards.
I can't definitively tell from the diagram what your problem is, but I've got a pretty good guess. This is actually something I've run into before. You may have defective ammeter.
As these early ammeters get old, their construction starts to decay. They do not have a "shunt resistor" design like modern ammeters do - so ALL the current output from the generator goes through the ammeter. The early designs didn't include safety devices like fusable links. Anyway, the resistance of the ammeter will change as it gets older, possibly shorting out. When it shorts out, it tends to fry the wiring harness. The reason you see it happen while the headlights are on is because the generator has to push a lot more current to supply that headlight load - all of which has to go through the ammeter. The ammeter heats up, starts to short out - and more and more current is pushed through the ammeter from the generator trying to keep up. If wired incorrectly, the fuse may blow but the lights will still remain on. If the car shuts off, the current ceases to flow - and the ammeter cools down again (until the lights are turned back on).
I suggest that you try eliminating the ammeter from the circuit for a short while. You can run a test in this configuration (including a seperate fuse for the headlight circuit) to see if that is the problem.
I also suggest that you consider including a fusable link in your wiring harness to keep you from burning it up. I have seen a number of trucks with this type of circuit that were parked simply because the ammeter fried the wiring. No one knew why it had burnt up - and replacing the wiring just fried the new wiring. So the truck was just parked and/or parted out.
It took a while for the engineers at GM to discover the fault in their design - after that they never used that style of circuit again. They used a different ammeter design and incorporated fusable links into their circuits.
