The heater box is right behind the metal panel on the passenger side--no room there for a stereo. If your car is not A/C equipped, there's a ton of room behind the fascia above the radio where the A/C vents would normally be.
You'll have to cut a panel to do this, of course. I had a couple of these though, neither in terrific condition, so I can always go back to stock if I want. Last time I checked, Bird Nest was selling decent replacements for about $100.
Remember that the wiring on these cars is getting fragile. Instead of tapping into a wire under the dash, I ran power from the battery directly. Good luck!
~Steve