I think you have the voltage readings for B and C swapped. No way the voltage goes back up. Suggestions below are with B and C switched.
A to C drops 0.55 volts. That is 10% of the voltage. Way too much. Clean up the connections on both ends and try again. If it still reads the same replace the wire.
B to C drops 0.31 volts. That is 5% of the voltage. Way too much. Clean up the connections on both ends and try again. If it still reads the same replace the dimmer switch.
B to D drops 0.62 volts. That is 3% of the voltage. Too much. Clean up the connections on both ends and try again. If it still reads the same replace the wire.
E to D drops 0.20 volts. That is 3% of the voltage. Too much. Clean up the connections on both ends and try again. If it still reads the same replace the wire.
H to G drops 0.63 volts. That is 10% of the voltage. Way too much. Clean up the connections on both ends and try again. If it still reads the same replace the wire.
So the net is that you are loosing 2.31 volts of your 6.34 available volts (36%) before it gets to the headlamps. Makes for dim lights.
'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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