technically it can be done. I am assuming your car is a 2000. you would have to grab the signal for your amps before the factory amp that is behind the radio since once the signal gets there it goes through crossovers which would make your system sound like crap. The 5x7 in the door and rear deck only play low frequencies and the 2 1/2 in the top of the door and rear play the rest. but if you grab the low level signal right out of the deck it should be a full range signal. you would have to not use a high-low converter. you would have to wire directly into those wires with RCAs. I have not tried this but it should work since your head unit doesn't have the seperate sub out like the 01 + units do.
If you are seriouly interest in doing this i could draw up the wireing for you. i should be able to pull all the wire colors at work.
If i am totally off someone correct me. i have done this in other cars that had factory amplified systems just never that year mustang.
one more thing: If you do this i would highly recommend getting a aftermarkt reverse harness to grab your leads since the factory wires are very small and would be hard to work worth, also if it doesn't work you can simply plug the factory wire back in and all is back to normall