Do you have a simple 12 volt test light like this one?
If you don't, it is a cheap and invaluable tool at a time like this.
On amazon.com and most automotive parts stores.
Dont buy the cheapest one you can find because then you are always wondering is my test light still working and can I trust the information it is providing me.
If you did have one the first test you would do is to connect the alligator clip to terminal 1 on the trailer connector and the end of the probe on terminal 4 and have someone step on the brakes.
If the test probe works you immediately know the wiring from the to and from the brake pedal is ok and can focus on other possible problems.
Dave M7040