Our X5 doesn't have PDC, so that's not a concern. But clearly must have the module as it recognizes the presence of the trailer when the plug is fitted.

The boat trailer has surge brakes - stainless steel disc from Tiedown Industries - that work very well. One time I tried to backe it up a hill with the connector out and the truck couldn't overcome the brake. The harder I pushed, the harder it fought. There's a blocking solenoid valve in the brake line for that reason.