is it a high spec, or low spec fiesta??
if its a high spec (ie xr2, or one with the larger dash) then you will have a din style wiring, with a switched live connected to the original stereo, and an earth, as well as the speaker connections

you'll need to take a perm live, ideally from the cig lighter, and then all is nice and easy! - you wont need to worry about the blue as has been said, as is norm for eleci arials / amp)

there may even be a wiring guide on xrtwo.com, or retrofiesta.co.uk i think it is!