Both headlamps blew at the same time ?

You want as close to the regulator as you can get. You still have a ballast? One of the terminals comes from the key, and branches off to feed the regulator. Probe them with key on and your meter grounded to find the highest reading

Then leave your probe on that terminal, and transfer the ground probe to battery POS

Another way is to clip to the blue field wire for a 12V source.

You are trying to discover the "end to end" voltage drop. You most certainly DO NOT want to probe under the dash, as this may eliminate the actual problem and lead you astray

The circuit path for a factory car is.............

Battery.........starter relay.......fuse link.......through the bulkhead........to ammeter......through ammeter........out ammeter..........to ignition switch connector........through switch........out switch connector..........back out through bulkhead.......and branch off to field, VR, igniton

EACH of those is a potential problem. Your most likely is the bulkhead connector, the ignition switch connector, and the switch

More rare, bad ammeter connections or problems in the ammeter. More rare yet is the factory wedled splice in the black ammeter wire.