you know, one thing I didn't think about, this unit has a "brake release circuit", that you can disconnect one set of wires and plug it into a particular normally empty 'molex' connector, and it will power the brake solenoid so you can push this truck, if that is the case, you will need to put the brake wires back into their proper connection, and then the unit may work correctly.
also to consider on this unit, there is a connection to the battery charger, that when the battery charger is plugged into 110vAC, the unit is completely disabled, and has no codes.
you might check to be sure the wires to the on-board charger, too.