I believe the '01+ Ford radios are double DIN units. '94 should be a single DIN. I would also think the electrical connectors changed along. I can say this for the Mustang but not the F150 or the other trucks, although my friend's 03 Explorer (or similar) had a different radio (DDIN, single disc, cassette, non-Mach system). Up until '01 the connectors were long and skinny, as well as coded as black and gray. The upgraded sound systems used a square black connector (8-pin). Around '01 the Mustang changed the harness to a square red and black plug. I believe one or two other changes occurred depending on the specific audio system and it might have even varied for the '03/04 year.
However it could still be very possible if this is the case. You will have to get the correct female harness to plug into the car, and then the correct male harness that plugs into the back of the Ford radio. Both of which they make and are readily available. You will just have to wire these two harnesses together.
The second thing you would have to do is get the radio trim bezel out of an '01-04 Mustang and replace it with the current one in your car.
Also the '01-04 DDIN radio is held in by two screws on the bottom and a plastic clip on the back of the radio which slides onto a metal track. The screws are covered up by the plastic trim. You might have to move (or remove) the current metal bracing on the inside.
Hope that helps.