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It most likely wiring issue if someone added their own trailer lights under drivers side rear of the Ranger, main rear light wiring harness
If the wires there have not be messed with then problem is the Multi-function switch(turn signal stalk)
Brake/turn use 1 wire for each side for rear bulbs
Front turns and dash turns are separate wires
Brake light 12volt(pedal pressed) travels from brake pedal switch to Multi-function switch and then out on the 2 wires to the rear
BUT, there is a separate brake light wire that travels to 3rd brake light on the cab, by-passing multi-function switch
So if 3rd brake lights works then brake pedal switch works
Try the 4-way flashers, they use a slightly different pathway in the multi-function switch, but the SAME 2 wires to the rear
Does Flash-To-Pass work, high beams when you pull stalk toward steering wheel, just checking functions
When you turn on a turn signal the multi-function switch disconnects the 1 rear wire from brake light wire on that side and connects it to the flasher module, and bulb starts to blink
The other sides brake light would work normally
Muli-function switch is accessed by removing the top and bottom "clam shell" covers from steering column
It has two screws/bolts holding it on, remove them first then unplug the connectors
Inspect the wires/connection
Plug them back in and check if brake lights in the rear both work now
In the diagram you will see a Dark Blue/White stripe wire is used for Left rear(drivers side) and a Brown/Yellow wire for Right Rear
If those wires get 12volt then bulb will light up if wire is OK
There is a Dark Blue/Orange wire from fuse 24 in Engine Bay fuse box that has 12volts all the time at the multi-function switch(for Flash-To-Pass)
You can use that as the 12v source and jumper it to one of the rear wires to test if wire is OK, bulb lights up
If both wires work then replace multi-function switch
And just FYI, fuses 3 and 7 in cab fuse box were only for factory trailer wiring harness they wouldn't effect rear brake/turn wires on the bed