I found having a couple of TEST ADAPTERS keeps me ahead of the game trying to figure out if it is a truck connector problem or trailer problem.
If the ADAPTERS work good plugged into the truck connector then I I know the problem is with the trailer.
A good resource for trailer wiring pointers is ETRAILER. Check out their wiring diagrams too...
I carry these two TEST ADAPTERS in my toolbox...
7-way ADAPTER Plug
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4-way ADAPTER Plug
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These are available from ETRAILER or AMAZON
I also always squirt some W40 into the truck connector side and work it around a few times to clean up the contact blades/pins etc before using.
Roy Ken