Well if you can't get the plug you will have to run the new speaker wire. If you do that you won't be able to use that amp because that smaller square plug that was in the back of your tape deck won't be pluged in to the HU or, anything for that matter, therefor it won't be tell the amp what to do. Then to hook up the amp to you HU you would normaly run the remote wire and RCA cables back to the amp, but the stock Ford amp doesn't have a place to plug in any of that. So that won't work. If your putting in a new sub and amp I would do it just like any car. Run the RCA's, the remote wire(from you HU), the power from the battery, and hook up a ground. This is offcorce after you manage to get your HU in. The wire kit that I got came with everything that I needed including the ant wire. Which runs right along with the speaker wires. From the wires that you unplug from the amp in the back, to the HU. The place that I got my wire kit was a local store called the Detail Center. It is in with another buissness called the Battery Box. I their # is 1-608-649-3746. I'm preaty sure that the two buissness have seperate phone #'s, but if you get the Battery Box don't freak out. Now when I got mine he had to order it and it took almost a week. So it wasn't real fast but I got it. Then I guess once he got it he would have to ship it out to you so it might take longer. It cost me like $31. The hardest part of the whole thing is getting the back panal off. Once that is off all you have to do is hook up the wire kit and ant wire back at the amp and run the wires down the right side of the vehical in that same spot that all the other wires are ran, snake it up through the dash and hook those wires to the wires from the HU and plug in the ant.
Like I said before. Good luck