Here's what you do, worked the same as my '71.
Remove the Dash Pad (roughly 8 screws along the bottom). The dash and the front metal plate will pull out easily. Then you can get to the speaker easily through the top.
I would recommend you replace it with a pair of speakers that mount in the same location instead of the one so you can get stereo. You can buy the set from any Mustang Shop (Paddock, National Depot, California Mustang, etc. or your local place). You will be happier.
Here's some examples: http://www.restoremustangs.com/Dash-Speaker-262.html
Many of them can be reached by laying on the floor of the car and looking up under the dash and pulling the speaker out that way. It's a tight fit but it can be done. Another way is to remove the glovebox liner and reach the speaker throught that way.