U did say din plug!!!!
Ok if im right u may have a factory amp fitted. Usually in sum of the older fords they were above the pax kick panel behind dash/left of glovebox area.
I bypassed one in an car i once owned, if i remember right the other plug that u are missing is plugged into this amp(these will be the speaker output wires btw). You can still use the halford iso connector wiring but youll have to extend the wires to reach the amp(no need to cut original wiring)???
If no joy mate ignore what i said before its cut n shut time!!!
Hope this helps.