. The largest diameter red and black wires probably are the A1 (Battery) and R6 (alternator feed). The 70 Plymouth manual will show exactly which cavity in the bulkhead connector they go through if that is a question.
On the steering column connector the heaviest red is the power feed from the main splice.
The heavy black is probably switched accessory feed. Need to check the book for your car to be more certain. It only gets power when the key is switched to run or accessory. It supplies power to the windshirld wiper switch, turn signals, and through 2 or 3 fuses in the fusebox the heater fan, radio, and maybe a few other options.
To find resistance in circuit using voltage drop, current must be flowing through the resistance.
I'm not clear on whether 10.8 V was between J1 and ground, or between J1 and Accessory (or whatever black it was ).
Also what was turned on, if anything.
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