Very hard to tell. I may have misled earlier - the wires were broken in the
"slack" bit below where the harness runs inside the hinge. Presumably this
"slack" is there to allow the wires to take up the movement of the tailgate
opening. The strange thing (to me - not being an autoelectrician who
probably sees these things daily) was that the whole wire was broken, casing
and all. In the past I've had wires break, on a steam iron, for example, but
that was the metal strands inside the plastic casing and from the outside it
looked perfect. My wife, on seeing the harness, said, "It looks like they've
been cut". So I  don't really know what happened. I suppose they could have
just become fatigued as some of the other wires running alongside the broken
ones had cracked plastic casings but the metal strands were ok. 4 years
seems a short time for fatigue to happen, though.

Steve