Can i connect the amplifier and subwoofer to the built ford fiesta radio

If your amp has rca jack inputs only you will need in-line filters to reduce the stock stereo speaker outputs down to less than 3 watts. If your amp has high power inputs you can use those. If your amp does not have a built-in crossover then you will need to install a low-pass crossover or filter before the amp so the only the bass comes thru or the sub woofer will have major distortion. Another problem is the amplifier needs a turn-on signal for the blue wire and most factory radios do not have one. That means you will have to tie that wire into the switched 12v wire from the vehicle so the amp comes on when the key is switched on. It can be done be done but I don't think you will be very impressed with the sub due to most factory stereos have a limited bass range. Most aftermarket head units have built-in crossovers and sub outputs. They also produce much better sound than the factory unit. So in other words yes you can.