1st, is the battery well charged? and does it have any sort of 'short to frame' that has less than 100k ohms?
2nd did you set up the controller with a handset or computer so the settings match the truck's specs?
3rd are you SURE this is the proper controller (part number), and not a 'looks and fits the same" controller.
Code 66, on a GE SX traction controller indicates (to me anyway) either a problem with the drive motor, power cables or the traction contactors, and would extremely rarely be the controller itself. if someone has changed the contactor assembly, it may be that the coils in the new assembly were not the proper type.
The book says a code 66 is "The field current exceeds the current
limit of the field transistor".
POSSIBLE CAUSES
Shorted field F1 to F2 *=F1 or F2 terminals shorted to battery positive (B+) or to battery negative (B-)
Binding line contactor *=Visually inspect tips, clean them with a
solvent. *=Confirm that contactor includes G248 coil.
Noise *=Missing or damaged horn suppressor
Confirm that all inductive loads/accessories are suppressed.

I know I have seen code 66 from power cables being routed too close to the control circuits, but that was intermittent and related to the traction load (up ramp set the code from EMF noise)

you did put the big cables for the armature and the little cables for the field?