Try this:
Key off.
Attach the multimeter ground probe to a good ground and leave it there for the rest of the test.
Remove the tail light fuse on the firewall.
Turn on the low beam headlights.
Measure and record the voltage available at...
... A. the terminal board low-beam connection near the front of the driver's side inner fender
... B. the low-beam connection on the dimmer switch
... C. the common connection on the dimmer switch
... D. the headlight connection on the headlight switch
... E. the power connection on the dashboard light switch. It's on the forward end of the switch and may be marked "AMM"
... F. the non-battery side of the ammeter
... G. the battery side of the ammeter
... H. the post on the stomp starter switch that the battery cable mounts on
When you are done, turn off the headlights, put the tail light fuse back in and post the results, A through H.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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