Could be the "in harness splice."
Or you may have multiple problems
You have wiring diagrams?
Download the closest shop manual, here is the '72:
http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download
I assume your car is a '71, here's a different take on diagrams:
http://www.mymopar.com/downloads/1971/71ValiantA.JPG
http://www.mymopar.com/downloads/1971/71ValiantB.JPG
Battery feed for pretty much everything comes in on "J" of the bulkhead: (See where it says "fuse link")
(page 8-147 of the 72 shop manual)
Then go to pg 8-150 a1-12R comes into the interior from "J" of the bulkhead, goes to the ammeter, comes out as R6A-12BK and goes to
The number one splice, near bottom of page, 8-150. This is a huge welded splice inside the harness, and CAN go bad, and feeds "a lot of stuff"
From the splice, top left, J1-12R is the main feed to the ignition switch
Middle left, R6A-12BK, goes to "P" on the bulkhead connector, eventually to the alternator output
Bottom left, H1-16V is the feed to the horn relay and buzzer (goes up to pg 8-150)
Top right, L1-16BK is the "B1" (battery 1) feed TO the headlight switch
middle right, R6A-12BK once again, is battery coming in from ammeter
bottom right, Q3-14R, is the 'hot' accessory feed to the fuse block.
So see if you have power at the "big red" at the column connector, going to the ign switch
See if the any of the fuses are "hot" key off.
If no to both of the above, I'd say the big splice has broke inside the harness. It is not the ammeter, because the headlights work.
It may be that you have a problem at the ignition switch/ connector AND at the fuse box as well, instead