Hy . You mentioned your tail lights, yes, there is a rubber seal between each tail light assembly and the body of the car, Make sure it is in one piece. If not, you can make new ones with rubber material, even inner tubes( car tires) ...work good(if not able to find the original seals or a piece of rubber big enough to cut a new seal )..., and make sure when tightening those small nuts, to do it in a cross pattern, as in bolting in lug nuts in a wheel, to make sure no water comes in.
About those bat cables, get a bean bag or something else and put the bat cables in it, those cables free flying around the trunk will chip off paint, you would be surprised to see how rust starts from those little nicks and scratches,add humidity(hot summer/wet winter)... rust is not welcome in a Datto.
Later____________________________