The previous post pretty much has it.
You can connect your
aftermarket radio speaker leads directly to the ones labeled as such on your wire harness adapter, those lead to the tweeter pods in the doors (front) and to the tweeters on the rear mach woofer box thing (rear).
Take all the Mach speakers themselves (the door woofers if present, the tweeters in front but not the tweeter housings, the whole rear enclosure if it's a coupe or just the speakers if it's a convertible) and put them on ebay there is nothing about them that is valuable in the absence of a fully OEM Mach system. The OEM Mach amps are specialized to the exact speakers they were built for and sound bad on any other speakers.
Get a set of 6x8" or 6.5" components for the front. Cheap ones installed correctly will perform better than expensive ones installed badly. I'm not going to recommend a brand because I work for a company that manufactures speakers. See below for an important note. There's a wire at the top of your door that connects to the Mach tweeter, now that supplies all the audio to your door. Connect that to the crossover which came with your components, or if they're super cheap and don't have a crossover then connect that to your speakers as directed in the instructions. Ignore the previous wiring for the door woofers, this is not useful anymore. Install the tweeter in the triangular tweeter pod. There are adapters for this that require minimal cutting (of the adapter) or you can make your own out of cardboard or aluminum or whatever you're comfortable working with. Some installers literally just hold the tweeter in place and fill the void with hot glue and backer rod. I don't happen to like that, but it works.
Get a set of 6x8" coaxials for the rear. If you have a coupe, you have to take the plastic trim panels off both sides of the rear window (the triangular window frame that surrounds the rear side windows) before lifting off the rear shelf cover, don't try to shortcut that unless you like having unsightly cracks to deal with on your shelf cover. Mount the coaxials in the existing cutouts. Connect them to the wires that previously supplied sound to the Mach tweeters, those are your full range speaker signals now. Ignore the previous woofer wiring, it's not used anymore.. in fact it should have gone away with the amps and box.
That's it. You can get infinitely more complex if you want, that's on you.
Note about speakers: Ford uses a slightly funky speaker size, 6x8" and unfortunately there are multiple standards for the exact shape and spacing of the mounting holes. If you want to make your life slightly easier when installing, shop for ones that have the kinda more square sides instead of the little ears for mounting. Or just get 6.5" round speakers and use adapter plates.
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