It all depends on the year Mustang and the year of the radio.
In 2004, Ford changed some of the plugs to just some of the radios and the cars. If the radio plug is the same, yes, it will work. Will it fit? It should.
My buddy had a Mustang, I believe a '99 and I had a 2003 Ranger MP3 CD player [NOT the 6CD/mp3] and it worked in his, and vice versa.
My Dad's Sport Trac  didnt work in my Ranger, as the plug is the different, but my mom's Sport Trac  DID work.
Do you have one that you are thinking of buying? Do some research on Visteon. They will have the same part numbers [hopefully] that you can compare.
To MY knowledge about the Ranger vs. Mustang, which is quite extensive [I owned both at the same time], it will work.
Just get a radio from a 2003 and older, and not a 2004, since the plugs might be different.