I know we're talking about a Sunny here, but I imagine it would be at least a similar procedure to earlier models...

it is a 4" speaker and in order to remove it, you need to basically gut the dashboard..

First of all remove the dashboard itself until only the metal frame and crashpad remain.
then, From underneath, undo the 8 screws or so holding down the crashpad and then remove the speaker. If you are going to replace it with the intention of installing a mono Centre Channel speaker, don't forget to take the left +ve and right -ve cables as your input.

Hope this helps.