Ford StreetKa Rear Speakers

[Bryank9]

Hey all, So in my StreetKa there is two panels, one behind each of the two front seats, if you cut the tabs on the carpet it will reveal some Brackets and speaker wire with a socket on the end, I am looking to see if anyone knows what speaker size fits directly into the brackets already in place as I have tried 5x7 as suggested in another Forum but no luck, I've been to Halfords and tried 6x9 with no luck, I'm not sure what to do next to be honest, Any help greatly appreciated