[MetroMPG]

Are you saying you go from all batteries "fully charged" to "unable to start your engine" in one day?

Sounds like the batteries are either too small (low capacity) or pretty worn out (low capacity) or the power draw of the stereo is pretty high. Can you estimate the power usage in watts of the stereo?

If you want your panel to be able to support the batteries at the same time you're using the stereo, you need to find a panel output at least equal to that total draw for the limited number of sun hours you have.