Aftermarket Amp and speakers?
Yes, you could make that work, but you'll need to MAKE your own harness, and you'll need a 4-channel amp with speaker-level line inputs.
You need this to plug into the radio
Amazon.com: Stereo Wire Harness OEM Ford Mustang 94 95 96 97 98 99 (car radio wiring installation parts): Electronics
and this to plug into the factory speaker harness
Amazon.com: Stereo Wire Harness Ford Mustang 94 95 96 97 98 99 (car radio wiring installation parts): Electronics
Basically, the OEM amp setup on 87-92 Mustangs uses speaker-level line inputs, so you are gonna do the same thing, only use an aftermarket amp that has speaker-level inputs. You'll just create a harness, and install it inline with the head unit and factory speaker harness.
You can even add the door speakers as long as you put a bass blocker on the dash speakers. That way all the low power is filtered out and the dash and door speakers are wired in parallel and doesn't reduce the load on the amp.
Would actually be a fun little project if you decide to do it. I can supply some advice i learned from my radio install. You can check the link i posted above, but the setup is slightly different...but same principle
But yes, it's completely possible to use the OEM head units, with an aftermarket amp and aftermarket speakers...even add a sub box. How well will it sound? No clue.