When the unit has been wired up, fit the weatherseal to the back of the socket, then fit the entire assembly to the mounting bracket. Align the screw holes in the socket with those in the bracket. Fit the securing bolts into the socket, fit and tighten the nuts at the back of the unit. Run the wire into the boot.
A caravan or trailer must, by law, be fitted with certain warning lights. These are wired up to ...