Is this a Mk7/7.5 Fiesta, or a newer Mk8??
First action would be to look and see if there is any water staining on the rearward most section of the headliner (does the car have a back of roof mounted antenna, or the usual Euro FRONT of roof mounted one??).
If dry and clean (or even if not), next, remove the tail light assembly and re-tape those most likely frayed and peeling factory tape swatches covering the openings in the unibody structure.
If still leaking after that, check the rubberized seam sealer material near the top of the hatch on the unibody side (outboard of the hinges) for pits, pores, or deterioration, and reseal, or heavy waterproof tape over them.
STILL water getting in?? Check the lower rearmost hatch area corners for too sparse or deteriorating seam sealer material and re-seal.