[outnumbered]

on mine i had to purchase the trailer module and the harness.on the left hand side there is as you have found a board with multi plugs already in and fuses that is the Rem. from the Pdf it states that the trailer module is already fitted in your car in mine it was not hence i had to pay £120.
if the trailer module is there it will be tucked up or in the panneling on the nearside rear side clost to the Rem. if not i would ring a volvo dealer and ask them if they are fitted as standard to your car.
from the 7/13 pin harness to the trailer module in . from the trailer module out to the spare slot on the Rem and a fuse has to be put in to activate it.
mike