My trailer is wired as a four flat, three lines and ground.

My knowledge of this is limited, but I guess what I want to know is if
there is any hope that my 81 vanagon uses the same light signal sequences
for turns and brakes, or do I need some kind of an adaptor to create the
signals that the trailer wants?

Hope I asked that right :)

Much obliged for any advice, as usual, I am in your debt.

Regards from Utah,

John