the biggest issue you are probably having is grounding , followed by dirty contacts.
under the front seat where the wiring enters the trailer is where to start for ground issues.
that's where i found the ground wires all twisted together and badly corroded.
Cleaned them up, and moved on.
then I took some scotchbrite and with the battery
out, and the trailer not
connected to the van, I cleaned all of the light
sockets on the entire trailer.When i replaced the bulbs I gave each a little coat of di-electric grease to prevent further corrosion.
I re-crimped most of the individual connections at the lights
, and my headaches went away.