Re: trailer wiring
It is similar to an open neutral in house wiring. Example - the kitchen lights won't work unless the toaster is depressed...!
Loss of a ground connection on a trailer can result in two lights working in series, i.e. - both lighting when only one should. Or in nothing lighting when it should. Depends on where the open connection is. And, yes, weird things can happen - always explainable when the circuit is traced out, but not at all obvious until then.
