I carry one of these 7-way testers that has the small LEDs on it in my tool box. This tells me the truck side is doing what it is suppose to do...
Then I know for sure it is on the trailer side...
This plugs into the 7-way receptacle on the back of the truck
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All of the DOT safety lights on the trailer are powered up by the truck 12VDC system.
Check your truck manual for the different locations for the various fuse panels... I think mine are in three different location... My main is just above the radiator under the hood... I have another one near the glove box inside the truck. I have another one behind the kick panel on the passenger side inside the truck...
The fuses for the 7-way and 4-way plugs usually have the word TOW in the description.
Roy Ken