They sell an insert that takes up the rest of the space left when installing an aftermarket radio in the OEM hole. You can also get a wiring harness so that you can just pop the old one out and the new one in. They have both the harness and the mounting plates at circuit city, best buy, stereo shops, online, etc.
there's a company called scosche that makes an adapter kit (model # FD1340) so you can put an aftermarket head unit in our 96-99 tauri. it replaces the entire black oval in your dash. you have to take the climate ctrl knobs out of the old one and move them over to the new one, and run a wire harness from the dash back to the trunk where the factory tuner / amp is. i got mine from crutchfield.com, they probably still sell them there.
also, some of the ford factory radios came with controls for a 6-disc changer you can mount in the trunk. you can either get a ford unit, or i believe there's a couple companies that make an aftermarket fit.