Its an odd size, like 4x10 or something. I've seen a newer model replacement from some of the parts vendors (Mustangs Unlimited?) that is the right size but fits two speakers in there.
I'll have to stay away from that then. I wouldn't want to distort the quality of my old AM radio, might loose The Score 670 from Chicago.
Boy do I need an upgrade!
Actually about ten years ago I put two 6x9's in the rear tray and bought one of those made to fit Mustang tape decks (Custom Autosound?, not sure). Worked for about seven years,,, then the needle fell off the dial (I can see it laying in there), the left channel went out, the tape deck quit, and it stuck on AM. Now I just hover between The Score on 670 and WGN 720 (when the Cubbies play). To think I though I was cool when I put that stuff in, now I'm back to an AM radio and a single 38yr. old speaker in the dash.
I'd avoid the factory speaker and just skip it. Most of the ones venders offer are designed to go along w/ good ol cassettes and such and not really be for power. I'd get a good brand of 6x9s for the back package tray, Fosgates would be my choice, and venders also sell kickpanels with holes made in them for i believe a 6.5"? or something to that extent.