Both headlamps blew at the same time ?

Here are the "big things"

Bulkhead connector

These are three for this problem.........

The red battery wire coming in, the black alternator wire going to the ammeter, and the IGN1 "run" wire

Ammeter itself and it's connections. These can be problem children right in the crimped / molded eyelets on the ends of the ammeter wires

The welded splice. Only way to check this is to pull the cluster and untape the black ammeter wire until you "get to" the splice.

If you have not done so, read the MAD article

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

Below, from that article is a simplified view of the main power

Power comes from the battery, to the start relay large stud, through the fusable link, and in through the BULKHEAD on the RED wire to the AMMETER. The power goes through the AMMETER, out on the BLACK ammeter wire to the WELDED SPLICE. Not shown, this branches off to the IGNITION SWITCH CONNECTOR, through the SWITCH, and back out the SWITCH CONNECTOR. From there, on the IGN1 "run" line, back out the BULKHEAD CONNECTOR

Also, of course, alternator charging current comes in through the black wire BULKHEAD CONNECTOR

ANY of these capitalized points can be causing this, or a combination of them