What you have there is a break (poor connection, etc) somewhere between the alternator output and the battery positive
Download a service manual if you don't have one
here
http://www.forabodiesonly.com/mopar/showthread.php?t=244981
and here. Also from the below link are simplified diagrams, sometimes easier to follow
http://www.mymopar.com/index.php?pid=31
here
http://www.mymopar.com/downloads/1968/68DartA.jpg
http://www.mymopar.com/downloads/1968/68DartB.jpg
Start by reading this excellent MAD Electrical article
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
and on that page is this nice simplified diagram:
which shows the main power distribution in these cars. Follow along. From the battery to the big stud on the starter relay, you go through the fuse link, and on the RED wire, through the bulkhead connector. This has to be good so far, or you would not have power inside the car.
Next we go to the ammeter and through the ammeter. We go out the ammeter on the BLACK wire to the 'welded splice.'
This is one suspect, but low on the list. This is a factory welded splice, up in the dash, which branches off and feeds headlight power, power to the fuse panel, and power to the ignition switch.
THE LAST BRANCH from the welded splice feeds off and back out through the bulkhead connector on BLACK. THIS IS YOUR MAIN SUSPECT!!!!
From the bulkhead we go on out to the ring terminal on the end of the black wire to alternator output. THIS IS your last suspect, and could be internally broken.
