Purnong

This might be out of left field but could be worth a look, Do both of you have towbars fitted? Reason I ask is I just discovered the dealer used Scotchlocks on my wiring loom to wire in the trailer plug, These things are notorious for creating a bad connection and even cutting the wires they are joined to It is possible that the earth or brake light wire might be cut if yours has been done like mine, Longshot but cheap to look