I don't 'have the assist' option but it is straight forward? Before should'ving on the dealer, pop the rear door pillar passenger side black plastic liner out and check for yourself, 7 or so screws and pry it away from the center then top down...
The OEM optional harness is wrapped with waxy fleece tape and there should be a 24~ inch loose wire with large flag label "PAM disable circuit" or somesuch printed on it clipped into the PAM module wiring plug, with the wire running back to the harness near the jumper plugs on the tail light.
The factory harness trailer lighting module gets its 12V from under the drivers feet next to the battery; just uses van brake/turn/tail light circuits to control separately powered trailer lighting circuits. Remember some 30' trailers are lit up like Christmas trees and the extra wire length and high current draw would not be healthy for the stock chassis wiring... So pretty sure there are no other wiring or dash board switches involved.
EDIT: some trailers (like mine) have a house battery that will accept charging current through the harness, though with the voltage drop from long wire runs even a 12-hour drive only partially recharges the trailer battery, so the 20 amp fused circuit has its limits.