Is sounds like your +5V supply from the controller is either dead or shorted to ground.
If you disconnect the main harness from the controller you should be able to check the +5V supply on Pin 5 of the 10 pin connector in relation to battery negative (-):
If you don't have 4~5V on pin 5, then the controller will probably need to be replaced.
However, if you do see 4~5V with the main harness disconnected, it could be a fault in the control harness, the throttle or either of the brake switches that is shorting the +5v to ground.
If you place a short piece of wire insulation on the end of the Positive meter probe
(or wrap some insulation or masking tape around it to make a tube) it will prevent the probe from touching against adjacent pins while you are checking for the +5V supply:
Hopefully one of the local
GM dealers will have a controller in stock as the controllers are physically the same for SP5, MP5 and MP Edge, however, the controllers require different firmware to be installed for each motor
(the USB programming lead will be required to reload the firmware).
Alan