Maybe this might help if you decide to tap into the radios wiring:
2011 Ford Fiesta Car Stereo Wiring Diagram
Car Radio Battery Constant 12v+ Wire: Yellow/Red
Car Radio Accessory Switched 12v+ Wire: Brown/White
Car Radio Ground Wire: Black/Blue
Car Radio Illumination Wire: Blue
Car Stereo Dimmer Wire: N/A
Car Stereo Power Antenna Trigger Wire: N/A
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: N/A
Car Audio Front Speakers Location: N/A
Left Front Speaker Positive Wire (+): White
Left Front Speaker Negative Wire (-): White/Brown
Right Front Speaker Positive Wire (+): White/Violet
Right Front Speaker Negative Wire (-): White/Orange
Car Audio Rear Speakers Size: N/A
Car Audio Rear Speakers Location: N/A
Left Rear Speaker Positive Wire (+): White/Green
Left Rear Speaker Negative Wire (-): Brown/Yellow
Right Rear Speaker Positive Wire (+): Brown/White
Right Rear Speaker Negative Wire (-): Brown/Blue