Having problems fitting Panasonic CQ-C1323NW car...

Question

Bought new leads (1 from car stereo wiring (a) to Panasonic lead (b), 1 new lead from Panasonic stereo(c)) but there's no power to the stereo when I attach them. There is only one plug with leads from Fiesta (a) but two with the lead (b) from that to the stereo lead (c). Sorry if I don't know the correct terms for these!