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: