The unit won’t switch on with 12v. What can I do?

Check that the 15amp blade fuse next to 12v input has not blown.
Check that the fuse in the 12v cable male socket is still intact.
Try a different 12v cable to make sure the cable is not damaged.
If the unit switches on with 220v but not with 12v on older version power supply, connect wiring on GB+ with V+ (220v and 12v then cannot be connected at the same time.
Check the voltage on the vehicle outlet socket.
Make sure that the power supply is not faulty.