Hi Hakon,
as the DVR starts when you connect power to USB, the DVR seems to be ok.
The steady blue led at the power converter means that it still senses the voltage of the battery, but does not sense switched power at the yellow cable. To check the power converter, you could connect the yellow wire directly to + at the battery. After some seconds, the blue led should start blinking and the DVR should start.
If the led starts blinking, then you need to check your connection of the yellow wire to switched power.
If the led does not start blinking, then is seems that your converter is faulty and you need a new one.
Regards,
Marc