Am I Supposed to See 12V at the Tail Lights?

Artum Snowbird wrote:
You basically did the right measurement and found the problem indirectly. When you measured the voltage across the light bulb filament itself, if your grounds were good, you would see the entire voltage dropped across one filament. When you read 10.4, that means the remaining voltage in the circuit is out there somewhere... and it is in the heavy brake/turn filament as I said. Measure the voltage across the brake turn filament. You will see it is half of the remaining voltage (13.6 - 10.4 = 3.2 1.6 on the brake/turn filament in that bulb, and 1.6 across the brake/turn filament on the other side of the vehicle.

This is a great learning experience/tutorial with insightful analysis on proper procedure for measuring DC circuits. Appreciate the collective wisdom.

Just to be clear once again . . . my test probes were applied directly to the bare wires of the positive and negative feeds coming into the tail light fixture at the back wall of the trailer. Earlier I had clipped those wires and stripped them back for such a measurement. What I did not do was measure the positive wire and the ground wire (which I will need to expose somehow to get a good reading). Nor did I measure the voltage across the bulb filament itself. I went straight for the wiring harness of the trailer. As I understand it, I incorrectly measured across only the positive and negative wires and ignored the ground.

After I collect another good cup of coffee, I'll back the truck up and connect again to my 7-pin and do this right way! Now, if I again obtain off-spec readings, I'll be back again for another tutorial!

Thanks to all.