Its either bad ground or you have the wires swapped around somewhere.


The classic ground fault usually shows up as tail ight or brake light
flashing at same time as indicators.
The light is being grounded back through the brake light and/or tail
light cct.

I would go back to basics and use a multimeter to find which wire does
which on the van.
Then power up the trailer lights from a PSU and determine which wire
is which on the trailer.

You then need to check the grounds.