Classic/Antique Car Repair: Low voltage to lights 39 Mercury

Question
1939 Mercury 4 dr still has 6 volt positive ground as original. Only change I see is sealed beam headlights.  Have 6.2 volts at battery but only 3.5 volts at headlights and taillights. I have shop manual with wiring diagrams but I am a novice.  Could you suggest a place to start to repair the problem?

Answer
The first question that I have  is the wiring harness original  or has it been altered or replaced? Let me know. To check the system check the voltage at the input side of the headlamp  switch with the lights on. The voltage there should be within .2 volts of battery. If ok check the voltage at the output side of the switch. Again this should be within .2 volts of the battery. If it is lower the switch is bad. Net check the dimmer switch the same way and the at the head lamps. This will point out where the voltage drop is. The final check is the ground connection for the headlights themselves.