Ok,Disconnect the  pin plug on the harness from pedal box to controller [found in front of controller] and see if it is corroded and apply some electrical grease to it and reconnect. With the rear tires off the ground try it in forward and if that doesn't let it work try moving the same harness back and forth at the point it enters the controller box by the tie down at the bottom because I have seen the wire in the harness break there. You will need to hold the accelerator down and wiggle the harness while having the key on and in forward or reverse. There are  wires in that harness  for switch and  black and white for ITS.
I did not replace the wiring harness to the ITS but I did ohm out the 4 wires.Sounds like the ITS is bad (I have bought new cheap ones that were shorted), the black/white wires are broken or the controller is bad.
MS2 switch is new and tests good for continuity.Have you checked the micro switch [ MS2] on the F/R switch ?
I was going to mention the F/R switch like HRC did. That switch can have issues like he mentioned. Its pretty easy to fix.
I've also seen the F/R wires not connected correctly.