it's a stock head unit?
My dads 93 had a bum radio. At it's worst, sound would come out of one speaker, and there were audible clicks or pops as you adjusted the volume (as well as bass, treb, etc.).
Weird thing was, if you increased the volume up to a certain point (a ridiculously loud point), all the speakers would come back to life. After i put in a Pioneer head unit, all the speakers worked and sounded great.
To remove the radio, go to an autoparts store and buy a DIN stereo removal kit (Ford specific i believe). Just looks like two U-shaped metal rods that slide into the 4 holes in the face of the stereo (or you may actually be able to fashion the tool yourself using some metal coat hangers, i dunno, didn't work for me

). Then the stock stereo just slides on out. From there you can either try your luck at a junkyard and get another stocker, or buy an aftermarket radio. It's not hard at all to install yourself, especially if you buy the wiring harnesses (makes all the connections plug-n-play). The aftermarket headunit will come with a metal cage that you slide into the opening for the radio. The cage has tabs that you have to bend outwards to secure it in the opening. Connect the new stereo and slide it in.
If you go the aftermarket route and decide to buy the harnesses, you can get them online. I got my from circuit city, I think they go under the name Metra? Regardless, you'll need two, one that plugs into the premium sound connectors behind the dash, and another that connects to the first harness and plugs into your new head unit.
So, say you bought a Panasonic radio. You'd have:
1)Ford premium sound (years :19XX-19XX) --> metra
and
2) Metra ---> Panasonic
Or if you're crafty with wiring, you can wire it all in yourself. Either using the connectors that were originally plugged into the head unit (which will utilize the amp(s) in the trunk), or theres another connector tucked away in the dash, that bypasses the amp(s) and sends the signal from your radio straight to the speakers. This is possible because the speakers in our cars are dual wired, there will be 4 wires running to each speaker instead of two. I'm pretty sure Ford made it this way so they only had to produce one audio harness. If i'm correct, lesser tauri had an audio system that utilized only the amp in the radio, and had no extra amps in the trunk, while the SHO, (both the premium sound and JBL equipped ones) had amps in the trunk.
Sorry, i rambled, i've been snowed in for quite some time.
But theres a brief audio insight into our cars.