12v @ 8a power multiplexer
Use an electromechanical relay. When the power adapter is not plugged-in the relay is unpowered and the normally-closed contact can route battery power to the load. When the adaptor is plugged-in the relay activates and disconnects the battery power in favour of the adapter power: -


NC contact to battery positive
NO contact to adapter positive
Common contact to load
Relay coil powered by presense of adapter