For troubleshooting purposes, the first thing I would do is set the unit to show data boxes and set one of those to display voltage ..... or look at the voltage in the system status screen.
If the voltage is low even before you try to crank the motor, you know you either have too small of wire or a bad connection somewhere causing the problem ..... do not use a glass type fuse holder. Use a marine spady type fuse. As some have mentioned, the connector on the back of the unit could also be the cause of voltage loss to the unit. There is a way to troubleshoot that also, but the first step is to see what the voltage is on the unit display without the motor running. With a fully charged battery and good wiring you should be seeing at least 12.2V or more