Couple of things you need to do, is look/ inspect the bulkhead connector and then CHECK it for drop.
Turn the key on, engine off. Put one probe on the battery, the other on the dark blue IGN run wire feeding the regulator and ballast. You are looking for a VERY low voltage, the lower the better.
Anything over .2V (two TENTHS of a volt) check the harness and try and improve things. If it's even higher, check REALLY carefully
The basic circuit path is the battery--starter relay--fuse link--through the bulkhead--to ammeter circuit--to ignition switch connector--through the switch and back OUT the switch--back OUT the switch connector on the dark blue--back through the bulkhead--to the ballast resistor.
Any connection along the way buy your main suspects in probable order are bulkhead connector, ignition switch connector, the switch, the ammeter circuit, ammeter connections, and last the welded splice taped up in the under--dash harness. This splice is tied to the ammeter circuit and branches off to feed the ignition switch, the headlights, the fuse box "hot" buss, and one or two other things.
Read this for an overview;
The simplified diagram down the page applies to many many years of cars and trucks
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
came from here
http://www.madelectrical.com/
If the above all checks out OK, check your charging voltage. You must REPAIR the above if there is excessive voltage drop before the charging voltage will be correct. A drop in the above test say it's 1/2 volt, will ADD that much to the charging voltage!!
So warm up the engine (regulator must be warm) get the battery "normalized" (ammeter centered) and run the engine RPM to simulate low/ medium cruise. Fast idle cam will usually do.
You are looking for an optimum right at the battery of 13.8-14.2V. Acceptable is 13.5-14.5. Anything higher / lower means checking into.
If it's a little high, confirm that the regulator is properly grounded. To do that, again check for voltage drop. Stick one probe on the battery negative post, the other right on the regulator mounting flange. Be sure to "stab" through any paint, rust, chrome. Again you are looking for a very low reading. Over .2V means the ground is not very good. The body, the block, and the battery neg. must all be at the same potential.
I believe its "wjar" on here suggests making a ground wire, bridging the ground bolts on the ignition module, and the regulator, and back to the battery.
The way I did mine was to buy a one foot "starter" cable at the store, IE "ring to ring." Bolt one end to the unused head holes on the back end of the driver's head. Bolt the other end to one of the master cylinder studs, using star washers.
Next turn on some accessories, headlights, 4x flasher, radio. With the engine still running to simulate low/ med. cruise, check the voltage at the battery, then at the alternator output stud. If there's more than 1/2--1 volt difference, you have resistance in the ammeter circuit or bulkhead connector, and need to track it down.
