solid vs stranded wire = primarily flexibility, but when electricity flows through a wire, it flows on the surface, not through the "core", so a wire of a given size made up of many smaller strands can carry more power than a solid wire, because the many strands have more surface area - the reason why battery cables and welding cables are stranded wire - it allows them to carry more power with less of that power being dissipated as heat, virtually all wire used with DC voltage is stranded
I'm surprised any solid wire is used there, but I suppose there was a reason - an ohm meter should work to track it in the bundle, especially if you have the wiring diagram to get an idea where it should go
Bill
