We're in the same boat Taylor! I just received those lights for my (your old) van and am planning on tying into the tail lights or the stock license plate lights....In the meantime i have removed the aluminess box and put the plate in the stock location.
I haven't dug into it yet....I just rigged up a functional 3rd brake light for an inspection (fingers crossed) for tomorrow. It may be a bit before I get back to to the license plate and additional stop/turn/tail lights....In basic trailer wiring I believe the tail light wire is brown. If your wired for trailer lights follow the brown back to where it ties into the vehicle wiring.