I have one of these testers in my RV tool box that connects to the truck side to tell if all is OK with the truck side. This helps when troubleshooting...
I got mine from AMAZON but have seen them at local auto parts places - also at ETRAILER.com
This is also a general schematic of how a typical trailer is setup using the 7-way trailer wiring connector. Good reading at ETRAILER.COM on how all of this is setup.
I always squirt WD40 in the trailer connector and move around several times before hooking to make sure the pins are making good connect.
You are going to have to start tracing out the wires if you don't find anything inside the marker lights... All of this wiring is controlled by the truck. Does not interconnect with any of the trailer 120VAC or 12VDC wiring except for connection to the trailer battery.
Roy Ken
