I just did this to my 88 mustang. the speaker wires are way the hell up in the dash separate from the pigtail you get the power and ground from. Ill take a picture of it and post it in a few min. the pigtail you get the power from has the same colored wires as the speaker wires so you think your hooking up the speaker wires but you’re not you just hooking up to a disconnected wire that use to go to the amp you took out. it’s easier to get to the actual speaker wires from the passenger side floor/side area then to get to it from the head unit hole. I put all my connections together over in the passenger side floorboard then pushed them back up and into the head unit area.