I would almost bet that the factory that made the lights had to apply them showing that they had been tested. As a supplier to Ford we had to put marks on lots of parts especially if you ever had an issue.
Say Ford got a trunk light that the supplier forgot to put the mercury in the switch and it did not work. So they make the mfg. of the light mark the lights when the mercury is installed and also that the light was tested.
Lots of fasteners get paint marks to show that the operator actually did their torque on a safety critical fastener.