Once again, a gentle lecture ---find yourself a factory service manual.
Here's the 72 manual posted the other day:
http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual
list of wiring diagrams start on 8-144
More stuff:
http://www.forabodiesonly.com/mopar/showthread.php?t=131932&highlight=manual
Go to page 8-151, find the steering column, wire no J1-12R battery and follow it down and to the right to a big diamond shape marked "1." This is known as the "no 1 in harness splice" in other words it IS an "in harness splice" that is factory welded and taped right inside the under-dash harness. THESE CAN FAIL
At the splice, you'll notice two more #12 wires, R6-12BK and R6A-12BK.
R6-12BK goes "off the page" to the left and to the diagram above, where it eventually ends up at terminal "P" of your bulkhead connector. THIS IS part of the main battery feed "into" the car
R6A-12BK goes up the original page TO THE AMMETER
From the ammeter, A1-12R comes down the page and "off" it to the left, and to the upper page where it eventually goes to "J" of the bulkhead connector.
BELOW are simplified diagrams from here:
http://www.mymopar.com/index.php?pid=31
Two schematics:
http://www.mymopar.com/downloads/1973/73ValiantA.jpg
http://www.mymopar.com/downloads/1973/73ValiantB.jpg
If you look at the above two schematics (the wires are not numbered) you can see that "J" of the bulkhead goes to the battery through the fuse link,
P of the bulkhead comes from the output of the alternator.
On schematic B if you follow the "big black" from P of the bulkhead off and up to the right you can see the way that this diagram depicts the "number one splice."
EXCEPT FOR the fuse link coming from the battery, there is NO fuse in this part of the circuit. If you have had a short on the BLUE lead (coming from your ignition) it is possible that there is internal damage to the harness, that is, you should untape it and inspect the big red and the blue back to where they terminate. It well could be that they are melted / shorted to something else in the harness.
So far as repair, same deal. Once you get the harness partly untaped, and confirm that there is no more damage, you can solder/ heat shrink or butt splice new wire in.
INSPECT the bulkhead connections for damage. It is likely that you will find such in this case, considering the "melt" that you have already found.
The blue by the way supplies the following, and once again, is NOT fused
ignition in run
regulator power/ sense
blue lead (power in "run") to the alternator field
electric choke if used
throttle solenoid if used
It MAY (don't remember) be tied in to original distributor retard solenoid.
THIS (blue) circuit can be problematic. If the bulkhead is damaged, and for that matter the entire path from blue (at ignition) back through bulkhead, to ignition switch connector---through switch--and main power to ignition switch--ammeter circuit--through bulkhead--to battery
IF this circuit is suffering, you will have VOLTAGE DROP across that section, which can affect ignition performance (low voltage) as well as cause OVERCHARGING because it is supplying (low) sense voltage to the voltage regulator.
Sorry if this sounds complictated, but it illustrates just why there are so many posts about problems with the bulkhead connector and ammeter circuit.
