Hi Diesel,
Is the main contactor coming in,truck will still light up the display even if this doesn't come in,obviously make sure you have 48v both sides of the contactor,or at the 2 main heavy fuses,as they're sealed you can't actually see tips,so will need to check with a meter,and have you also checked the small microswitch on the side of the emergency disconnect?
as a general rule, negative is supplied through the controller.