Control unit for second battery and lights (J328)
The control unit for second battery and lights is required to switch the power from the main battery to the auxiliary battery during an engine starting procedure, this is so power to the lights and other necessary electrical consumers (such as the instrument cluster) is not interrupted during the engine starting phase. It also therefore ensures that primary battery is used solely to crank the engine.
This component has a fuse visible on the top of it.
If you lose power to the radio and/or instrument cluster when the engine is restarting then it is likely the auxiliary battery is flat.