Was it a mach system (3 amps) or a premium sound system (1 amp) If it is a premium system and you are not using the stock amp you dont need to use the square black plug. That actually feeds the amp. If you look in the compartment to the right of the radio you will see a junction of plugs that look like the flat one that supplies the power and such.(this goes right to the speakers) If you unplug this, you can then use the stock ford radio adapter (2 flat plugs). One will plug in to supply the power and ground.(you have this one already) and the other will plug into the part that you unplugged from the right. That should get you all 4 speakers running just off the aftermarket head unit. Now if you have a mach system, totally disregard all that Ive said. If you have a premium sound and are still using the stock amp then yes, you need to use that square connector, and I cant tell you why if worked before and not now.