Brake lights work on trailer but not on van
I have found using the hazard lights best way to make these tests for trailer wiring on most vehicles. That way you don't need to rely on clear communication from someone else, especially the untreated kind.
Places like autozone will come out and take a look..?
But, yes, if trailer brake lights work...probably not switch...
this is a sample video, but I'm sure you could find one for your specific vehicle