The panel you mention holds the dash, not the dash itself. The dash is still put together in the same way as the Breva one.
Remember you have to unlock the multiway connectors before you can pull them off.
Can someone explain why a poor battery would give the symptom of 13v across the battery and under 12v displayed at the dash at the same time, with only the ignition switched on? Yes if it dies when you press the start button, but the battery volts would die too.
My two cents.
I don't really think your issue is the battery, but it could be. The easiest way I can think to explain it is this. Say you have a battery, and that battery has 12 volts measured at the battery. I wire up to the battery a light, using 5 feet of wire and a light of a given wattage. When I turn the light on I should still have 12 volts at the battery. But if I measure the voltage of the battery at the light I will have less voltage. This is from the resistance of the length of wire and the draw of the light. If I have a higher wattage bulb I will show less voltage measured at the light. If I increase the length of the wire I will show less voltage at the light. The voltage at the battery does not change.
Another thing that could cause a larger difference between the two measurements is if your battery is weak and can't overcome the extra loads put on it by the above referenced aspects. A weak battery may have less amperage to compensate for the extra wattage or resistance. So the remotely measure voltage is lower then it should be.
The Dash is showing you the voltage as measured remotely, The current draw at the point of measuring and any resistance due to wire length and bad connections will cause the dash to show less voltage then what a meter directly at the battery will show. In my experience, some difference is normal. Yours sounds like it has some excessive voltage drop, likely just a bad connection.
I hope this helps someone.